Teaching indefinite integrals that require special-casing The Next CEO of Stack OverflowWhat do you do when you realize mid-lecture that your lesson plan is not working?Should $varphi$ be monotone in the integration by substitution?Teaching measurement to 4th grade students by projects?Students strictly follow the steps and notations in sample problems without understanding themTeaching Models for Mathematics (like 5 E's in Science)Struggles with Teaching HS Mathteach that $frac10$ not defined properlyWhat is the ideal teaching style for Calculus exercises only?“Basic ideas” as a concept in teaching maths?Should young math students be taught an abstract concept of form?

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Teaching indefinite integrals that require special-casing



The Next CEO of Stack OverflowWhat do you do when you realize mid-lecture that your lesson plan is not working?Should $varphi$ be monotone in the integration by substitution?Teaching measurement to 4th grade students by projects?Students strictly follow the steps and notations in sample problems without understanding themTeaching Models for Mathematics (like 5 E's in Science)Struggles with Teaching HS Mathteach that $frac10$ not defined properlyWhat is the ideal teaching style for Calculus exercises only?“Basic ideas” as a concept in teaching maths?Should young math students be taught an abstract concept of form?










17












$begingroup$


I encountered the following concern when teaching indefinite integrals. I believe that many of us may overlook this. May I be wrong?



Let's consider the following example.



Find the indefinite integral
$$
I=intdfracdxxsqrtx^2-1.
$$

Some of my students gave the following answer.



Let $t=1/x$ then $dx=-1/t^2dt$, so we get
$$
I=intdfrac-1/t^2dtfrac1tsqrtfrac1t^2-1=intdfrac-dtsqrt1-t^2=-arcsinleft(tright)+C=-arcsinleft(frac1xright)+C.
$$



Sometimes, I accept this answer since it gives a quick general antiderivative.
However, the problem here is that we should write
$$
intdfrac-1/t^2dtfrac1tsqrtfrac1t^2-1=intdfractrighttsqrt1-t^2.
$$

Then we end up with the answer
$$
intdfracdxxsqrtx^2-1=begincases
-arcsinleft(dfrac1xright)+C & textfor x>1,\
arcsinleft(dfrac1xright)+C & textfor x<-1.
endcases
$$

In your teaching practice, how would you usually proceed?



PS. We may encounter the same issue in many other problems. For example, find $intsqrt1-x^2dx$. Then if we let $x=sinleft(tright)$ then
$sqrt1-sin^2left(tright)$ should be $left|cosleft(tright)right|$. So now we need to explain a bit here to our naive students. Of course, avoiding these kinds of problems is the quickest way to make our teaching job easier. However, we need to prepare a good way of explanining or handing these types of problems. That's what I want to know.










share|improve this question











$endgroup$







  • 2




    $begingroup$
    Seems to me this is more of a general simplification/substitution issue, and the fact that the simplification appears within an integral is a side issue.
    $endgroup$
    – Acccumulation
    Mar 25 at 16:44






  • 8




    $begingroup$
    The correct answer should in fact have two different arbitrary constants, one for each connected component of the domain.
    $endgroup$
    – Javier
    Mar 25 at 19:06






  • 2




    $begingroup$
    By the way, there are so many equiv ways ... Wolfram Alpha gives a form of the arctangent, Sage/Maxima says your answer but with absolute value of 1/x, and Sympy gives a nice cases result including I*arccosh(1/x). And it looks like arc secant :)
    $endgroup$
    – kcrisman
    Mar 26 at 1:49










  • $begingroup$
    Very nice question. Inattention to this detail is indeed a shortcoming of many texts and many of my own lectures. Probably adding a condition ($x>1$ or $x<-1$) to focus attention on one case is the smart solution to not overwhelm students and yet be true to detail. Some texts are more careful than others...
    $endgroup$
    – James S. Cook
    Mar 27 at 1:44










  • $begingroup$
    Obligatory xkcd
    $endgroup$
    – Wildcard
    Mar 27 at 10:36















17












$begingroup$


I encountered the following concern when teaching indefinite integrals. I believe that many of us may overlook this. May I be wrong?



Let's consider the following example.



Find the indefinite integral
$$
I=intdfracdxxsqrtx^2-1.
$$

Some of my students gave the following answer.



Let $t=1/x$ then $dx=-1/t^2dt$, so we get
$$
I=intdfrac-1/t^2dtfrac1tsqrtfrac1t^2-1=intdfrac-dtsqrt1-t^2=-arcsinleft(tright)+C=-arcsinleft(frac1xright)+C.
$$



Sometimes, I accept this answer since it gives a quick general antiderivative.
However, the problem here is that we should write
$$
intdfrac-1/t^2dtfrac1tsqrtfrac1t^2-1=intdfractrighttsqrt1-t^2.
$$

Then we end up with the answer
$$
intdfracdxxsqrtx^2-1=begincases
-arcsinleft(dfrac1xright)+C & textfor x>1,\
arcsinleft(dfrac1xright)+C & textfor x<-1.
endcases
$$

In your teaching practice, how would you usually proceed?



PS. We may encounter the same issue in many other problems. For example, find $intsqrt1-x^2dx$. Then if we let $x=sinleft(tright)$ then
$sqrt1-sin^2left(tright)$ should be $left|cosleft(tright)right|$. So now we need to explain a bit here to our naive students. Of course, avoiding these kinds of problems is the quickest way to make our teaching job easier. However, we need to prepare a good way of explanining or handing these types of problems. That's what I want to know.










share|improve this question











$endgroup$







  • 2




    $begingroup$
    Seems to me this is more of a general simplification/substitution issue, and the fact that the simplification appears within an integral is a side issue.
    $endgroup$
    – Acccumulation
    Mar 25 at 16:44






  • 8




    $begingroup$
    The correct answer should in fact have two different arbitrary constants, one for each connected component of the domain.
    $endgroup$
    – Javier
    Mar 25 at 19:06






  • 2




    $begingroup$
    By the way, there are so many equiv ways ... Wolfram Alpha gives a form of the arctangent, Sage/Maxima says your answer but with absolute value of 1/x, and Sympy gives a nice cases result including I*arccosh(1/x). And it looks like arc secant :)
    $endgroup$
    – kcrisman
    Mar 26 at 1:49










  • $begingroup$
    Very nice question. Inattention to this detail is indeed a shortcoming of many texts and many of my own lectures. Probably adding a condition ($x>1$ or $x<-1$) to focus attention on one case is the smart solution to not overwhelm students and yet be true to detail. Some texts are more careful than others...
    $endgroup$
    – James S. Cook
    Mar 27 at 1:44










  • $begingroup$
    Obligatory xkcd
    $endgroup$
    – Wildcard
    Mar 27 at 10:36













17












17








17


4



$begingroup$


I encountered the following concern when teaching indefinite integrals. I believe that many of us may overlook this. May I be wrong?



Let's consider the following example.



Find the indefinite integral
$$
I=intdfracdxxsqrtx^2-1.
$$

Some of my students gave the following answer.



Let $t=1/x$ then $dx=-1/t^2dt$, so we get
$$
I=intdfrac-1/t^2dtfrac1tsqrtfrac1t^2-1=intdfrac-dtsqrt1-t^2=-arcsinleft(tright)+C=-arcsinleft(frac1xright)+C.
$$



Sometimes, I accept this answer since it gives a quick general antiderivative.
However, the problem here is that we should write
$$
intdfrac-1/t^2dtfrac1tsqrtfrac1t^2-1=intdfractrighttsqrt1-t^2.
$$

Then we end up with the answer
$$
intdfracdxxsqrtx^2-1=begincases
-arcsinleft(dfrac1xright)+C & textfor x>1,\
arcsinleft(dfrac1xright)+C & textfor x<-1.
endcases
$$

In your teaching practice, how would you usually proceed?



PS. We may encounter the same issue in many other problems. For example, find $intsqrt1-x^2dx$. Then if we let $x=sinleft(tright)$ then
$sqrt1-sin^2left(tright)$ should be $left|cosleft(tright)right|$. So now we need to explain a bit here to our naive students. Of course, avoiding these kinds of problems is the quickest way to make our teaching job easier. However, we need to prepare a good way of explanining or handing these types of problems. That's what I want to know.










share|improve this question











$endgroup$




I encountered the following concern when teaching indefinite integrals. I believe that many of us may overlook this. May I be wrong?



Let's consider the following example.



Find the indefinite integral
$$
I=intdfracdxxsqrtx^2-1.
$$

Some of my students gave the following answer.



Let $t=1/x$ then $dx=-1/t^2dt$, so we get
$$
I=intdfrac-1/t^2dtfrac1tsqrtfrac1t^2-1=intdfrac-dtsqrt1-t^2=-arcsinleft(tright)+C=-arcsinleft(frac1xright)+C.
$$



Sometimes, I accept this answer since it gives a quick general antiderivative.
However, the problem here is that we should write
$$
intdfrac-1/t^2dtfrac1tsqrtfrac1t^2-1=intdfractrighttsqrt1-t^2.
$$

Then we end up with the answer
$$
intdfracdxxsqrtx^2-1=begincases
-arcsinleft(dfrac1xright)+C & textfor x>1,\
arcsinleft(dfrac1xright)+C & textfor x<-1.
endcases
$$

In your teaching practice, how would you usually proceed?



PS. We may encounter the same issue in many other problems. For example, find $intsqrt1-x^2dx$. Then if we let $x=sinleft(tright)$ then
$sqrt1-sin^2left(tright)$ should be $left|cosleft(tright)right|$. So now we need to explain a bit here to our naive students. Of course, avoiding these kinds of problems is the quickest way to make our teaching job easier. However, we need to prepare a good way of explanining or handing these types of problems. That's what I want to know.







teaching teacher-preparation






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Mar 25 at 22:05







Hoa

















asked Mar 25 at 12:31









HoaHoa

1396




1396







  • 2




    $begingroup$
    Seems to me this is more of a general simplification/substitution issue, and the fact that the simplification appears within an integral is a side issue.
    $endgroup$
    – Acccumulation
    Mar 25 at 16:44






  • 8




    $begingroup$
    The correct answer should in fact have two different arbitrary constants, one for each connected component of the domain.
    $endgroup$
    – Javier
    Mar 25 at 19:06






  • 2




    $begingroup$
    By the way, there are so many equiv ways ... Wolfram Alpha gives a form of the arctangent, Sage/Maxima says your answer but with absolute value of 1/x, and Sympy gives a nice cases result including I*arccosh(1/x). And it looks like arc secant :)
    $endgroup$
    – kcrisman
    Mar 26 at 1:49










  • $begingroup$
    Very nice question. Inattention to this detail is indeed a shortcoming of many texts and many of my own lectures. Probably adding a condition ($x>1$ or $x<-1$) to focus attention on one case is the smart solution to not overwhelm students and yet be true to detail. Some texts are more careful than others...
    $endgroup$
    – James S. Cook
    Mar 27 at 1:44










  • $begingroup$
    Obligatory xkcd
    $endgroup$
    – Wildcard
    Mar 27 at 10:36












  • 2




    $begingroup$
    Seems to me this is more of a general simplification/substitution issue, and the fact that the simplification appears within an integral is a side issue.
    $endgroup$
    – Acccumulation
    Mar 25 at 16:44






  • 8




    $begingroup$
    The correct answer should in fact have two different arbitrary constants, one for each connected component of the domain.
    $endgroup$
    – Javier
    Mar 25 at 19:06






  • 2




    $begingroup$
    By the way, there are so many equiv ways ... Wolfram Alpha gives a form of the arctangent, Sage/Maxima says your answer but with absolute value of 1/x, and Sympy gives a nice cases result including I*arccosh(1/x). And it looks like arc secant :)
    $endgroup$
    – kcrisman
    Mar 26 at 1:49










  • $begingroup$
    Very nice question. Inattention to this detail is indeed a shortcoming of many texts and many of my own lectures. Probably adding a condition ($x>1$ or $x<-1$) to focus attention on one case is the smart solution to not overwhelm students and yet be true to detail. Some texts are more careful than others...
    $endgroup$
    – James S. Cook
    Mar 27 at 1:44










  • $begingroup$
    Obligatory xkcd
    $endgroup$
    – Wildcard
    Mar 27 at 10:36







2




2




$begingroup$
Seems to me this is more of a general simplification/substitution issue, and the fact that the simplification appears within an integral is a side issue.
$endgroup$
– Acccumulation
Mar 25 at 16:44




$begingroup$
Seems to me this is more of a general simplification/substitution issue, and the fact that the simplification appears within an integral is a side issue.
$endgroup$
– Acccumulation
Mar 25 at 16:44




8




8




$begingroup$
The correct answer should in fact have two different arbitrary constants, one for each connected component of the domain.
$endgroup$
– Javier
Mar 25 at 19:06




$begingroup$
The correct answer should in fact have two different arbitrary constants, one for each connected component of the domain.
$endgroup$
– Javier
Mar 25 at 19:06




2




2




$begingroup$
By the way, there are so many equiv ways ... Wolfram Alpha gives a form of the arctangent, Sage/Maxima says your answer but with absolute value of 1/x, and Sympy gives a nice cases result including I*arccosh(1/x). And it looks like arc secant :)
$endgroup$
– kcrisman
Mar 26 at 1:49




$begingroup$
By the way, there are so many equiv ways ... Wolfram Alpha gives a form of the arctangent, Sage/Maxima says your answer but with absolute value of 1/x, and Sympy gives a nice cases result including I*arccosh(1/x). And it looks like arc secant :)
$endgroup$
– kcrisman
Mar 26 at 1:49












$begingroup$
Very nice question. Inattention to this detail is indeed a shortcoming of many texts and many of my own lectures. Probably adding a condition ($x>1$ or $x<-1$) to focus attention on one case is the smart solution to not overwhelm students and yet be true to detail. Some texts are more careful than others...
$endgroup$
– James S. Cook
Mar 27 at 1:44




$begingroup$
Very nice question. Inattention to this detail is indeed a shortcoming of many texts and many of my own lectures. Probably adding a condition ($x>1$ or $x<-1$) to focus attention on one case is the smart solution to not overwhelm students and yet be true to detail. Some texts are more careful than others...
$endgroup$
– James S. Cook
Mar 27 at 1:44












$begingroup$
Obligatory xkcd
$endgroup$
– Wildcard
Mar 27 at 10:36




$begingroup$
Obligatory xkcd
$endgroup$
– Wildcard
Mar 27 at 10:36










5 Answers
5






active

oldest

votes


















13












$begingroup$

I'd avoid giving problems like that to students first learning indefinite integrals (either by not asking it at all, or specifying the range x>1 in the question). It's a subtle algebraic trap, and if the goal is to teach students the mechanics of integration, it's going to be distracting rather than helpful.



It might be an interesting question in a more advanced class, or as a question which is marked as difficult where students are expected (or told) to investigate their answer more carefully. (For instance, graphing the functions will quickly reveal that there's a problem with the first solution, and looking at the graphs is probably enough to figure out what the fixed solution should look like, though figuring out why might take students a while.)






share|improve this answer









$endgroup$




















    10












    $begingroup$

    This is a hard question, because students are so used to manipulation of this kind. I have found you are right that absolute values can cause the worst of these examples.



    Here is an example I ran into recently, which I hope will help your thinking. Observe that there are two different limits here:
    $$lim_xtopminfty fracxsqrtx^2+1 = pm 1$$



    The "usual" way to proceed with these (informally, in many texts nowadays) is to divide numerator and denominator by the highest power, so:



    $$lim_xtopminfty fracxcdot 1/xsqrt(x^2+1)cdot 1/x^2 = lim_xtopminfty frac1sqrt1+1/x^2=1$$



    But of course bringing the $1/x$ inside the root like that is the same invalid manipulation you are mentioning.



    In this case, we actually talked through it at an even more naive level, not more rigorous! Namely, as $xto -infty$, the numerator is negative and the denominator is positive. So the overall answer must be negative, no matter what the manipulation says. (You can graph it for them too.)



    So in your case, I would go more naive as well. Do a very rough sketch of $arcsin(1/x)$ (you can basically do this by drawing $-arcsin(x)$ and then "flipping over $x=1$ to infinity"), and then ask them whether this function is increasing or decreasing. When $x<-1$ it should be increasing (in fact, it should be increasing on the whole domain), so its derivative should be positive (by whichever numbering of the fundamental theorems of calculus you like). But $frac1xsqrtx^2-1$ is definitely negative there.



    enter image description here



    Now you can explain why you are picky, instead of just being picky because of some "dumb" $|x|$ thing students may find to be a little too abstract.




    Another answer brings up the question of whether this is a good question at all. But I think it is reasonable. What you may want to do, though, is find a way to discuss this "naturally", i.e. using the disconnect between what people write and then if they see what seems to be a "wrong" answer in the back of a book or something. Taking it as a first example in class probably will not register with them unless they are quite good at the concepts of calculus (not just mechanics), whereas pointing out why it is wrong/right should be better. (On another note, presumably there are branch cut issues here as well but presumably your class isn't ready for that!)






    share|improve this answer









    $endgroup$












    • $begingroup$
      And perhaps, one should be asking why in the first place students are "so used to manipulation of this kind". In my experience, this is a product of bad mathematical pedagogy, because the syllabus is often designed around 'how to do X and Y' rather than 'why should X or Y be true?'... The first question a good student should ask is "Why?". Your example is a good one to illustrate the error here, which is the failure to understand precisely what $sqrtx$ is defined as.
      $endgroup$
      – user21820
      Mar 27 at 8:30










    • $begingroup$
      Well, my point of view is that, in one semester, I can't fix 12 years of students being taught math is only manipulation. (Not that every student has only had this experience, but enough will have had it that I need to make it a part of my planning.) So I do my best to get them to start thinking along that road at every opportunity.
      $endgroup$
      – kcrisman
      Mar 27 at 13:19










    • $begingroup$
      Yup I understand that. =)
      $endgroup$
      – user21820
      Mar 27 at 13:22


















    9












    $begingroup$

    You are right to be concerned that the students are "missing something", but IMO the real problem here is that the question is completely artificial.



    In any application of this type of integral, most likely $x$ will be known to be either positive or negative, but not both, and only one part of the "either-or" answer would apply. And there had better be a good reason why the rest of the problem needs $x$ to be negative, when it could have been replaced by $-x$ right from the start!



    The same is true for the more common case of the indefinite integral of $1/x$ when $x < 0$, of course.






    share|improve this answer









    $endgroup$




















      6












      $begingroup$

      To me, the key point here is that the integral runs over a singularity. If you naively calculates a definite form that runs over the singularity you get the wrong answer. This is something I have done enough so that I have taught myself to be careful in this case.



      I am more a physicist than a mathematician, so what I care about is the connection to a practical situation, rather than the formal manipulation of symbols. If you or the students are of a similar inclination, then the presence of the singularity is what tips you off.



      As you know, different students respond well to different approaches, so I mention this one so that you might add it to your arsenal.






      share|improve this answer









      $endgroup$




















        1












        $begingroup$

        It is interesting to note that if we instead write the antiderivative as $I=-arctanleft(frac1sqrtx^2-1right)$, then this form is valid for both $x<1$ and $x>1$. In other words, using this arctan representation, we avoid a need for a piecewise representation for the antiderivative.



        This problem is well suited for formative assessment. I would give full credit for an answer of the form $I=-arcsinleft(frac1xright)$, and then use this as a launching point for a discussion of these more involved issues that you have uncovered, depending on the mathematical maturity of the student.






        share|improve this answer









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          5 Answers
          5






          active

          oldest

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          5 Answers
          5






          active

          oldest

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          13












          $begingroup$

          I'd avoid giving problems like that to students first learning indefinite integrals (either by not asking it at all, or specifying the range x>1 in the question). It's a subtle algebraic trap, and if the goal is to teach students the mechanics of integration, it's going to be distracting rather than helpful.



          It might be an interesting question in a more advanced class, or as a question which is marked as difficult where students are expected (or told) to investigate their answer more carefully. (For instance, graphing the functions will quickly reveal that there's a problem with the first solution, and looking at the graphs is probably enough to figure out what the fixed solution should look like, though figuring out why might take students a while.)






          share|improve this answer









          $endgroup$

















            13












            $begingroup$

            I'd avoid giving problems like that to students first learning indefinite integrals (either by not asking it at all, or specifying the range x>1 in the question). It's a subtle algebraic trap, and if the goal is to teach students the mechanics of integration, it's going to be distracting rather than helpful.



            It might be an interesting question in a more advanced class, or as a question which is marked as difficult where students are expected (or told) to investigate their answer more carefully. (For instance, graphing the functions will quickly reveal that there's a problem with the first solution, and looking at the graphs is probably enough to figure out what the fixed solution should look like, though figuring out why might take students a while.)






            share|improve this answer









            $endgroup$















              13












              13








              13





              $begingroup$

              I'd avoid giving problems like that to students first learning indefinite integrals (either by not asking it at all, or specifying the range x>1 in the question). It's a subtle algebraic trap, and if the goal is to teach students the mechanics of integration, it's going to be distracting rather than helpful.



              It might be an interesting question in a more advanced class, or as a question which is marked as difficult where students are expected (or told) to investigate their answer more carefully. (For instance, graphing the functions will quickly reveal that there's a problem with the first solution, and looking at the graphs is probably enough to figure out what the fixed solution should look like, though figuring out why might take students a while.)






              share|improve this answer









              $endgroup$



              I'd avoid giving problems like that to students first learning indefinite integrals (either by not asking it at all, or specifying the range x>1 in the question). It's a subtle algebraic trap, and if the goal is to teach students the mechanics of integration, it's going to be distracting rather than helpful.



              It might be an interesting question in a more advanced class, or as a question which is marked as difficult where students are expected (or told) to investigate their answer more carefully. (For instance, graphing the functions will quickly reveal that there's a problem with the first solution, and looking at the graphs is probably enough to figure out what the fixed solution should look like, though figuring out why might take students a while.)







              share|improve this answer












              share|improve this answer



              share|improve this answer










              answered Mar 25 at 13:27









              Henry TowsnerHenry Towsner

              7,1752350




              7,1752350





















                  10












                  $begingroup$

                  This is a hard question, because students are so used to manipulation of this kind. I have found you are right that absolute values can cause the worst of these examples.



                  Here is an example I ran into recently, which I hope will help your thinking. Observe that there are two different limits here:
                  $$lim_xtopminfty fracxsqrtx^2+1 = pm 1$$



                  The "usual" way to proceed with these (informally, in many texts nowadays) is to divide numerator and denominator by the highest power, so:



                  $$lim_xtopminfty fracxcdot 1/xsqrt(x^2+1)cdot 1/x^2 = lim_xtopminfty frac1sqrt1+1/x^2=1$$



                  But of course bringing the $1/x$ inside the root like that is the same invalid manipulation you are mentioning.



                  In this case, we actually talked through it at an even more naive level, not more rigorous! Namely, as $xto -infty$, the numerator is negative and the denominator is positive. So the overall answer must be negative, no matter what the manipulation says. (You can graph it for them too.)



                  So in your case, I would go more naive as well. Do a very rough sketch of $arcsin(1/x)$ (you can basically do this by drawing $-arcsin(x)$ and then "flipping over $x=1$ to infinity"), and then ask them whether this function is increasing or decreasing. When $x<-1$ it should be increasing (in fact, it should be increasing on the whole domain), so its derivative should be positive (by whichever numbering of the fundamental theorems of calculus you like). But $frac1xsqrtx^2-1$ is definitely negative there.



                  enter image description here



                  Now you can explain why you are picky, instead of just being picky because of some "dumb" $|x|$ thing students may find to be a little too abstract.




                  Another answer brings up the question of whether this is a good question at all. But I think it is reasonable. What you may want to do, though, is find a way to discuss this "naturally", i.e. using the disconnect between what people write and then if they see what seems to be a "wrong" answer in the back of a book or something. Taking it as a first example in class probably will not register with them unless they are quite good at the concepts of calculus (not just mechanics), whereas pointing out why it is wrong/right should be better. (On another note, presumably there are branch cut issues here as well but presumably your class isn't ready for that!)






                  share|improve this answer









                  $endgroup$












                  • $begingroup$
                    And perhaps, one should be asking why in the first place students are "so used to manipulation of this kind". In my experience, this is a product of bad mathematical pedagogy, because the syllabus is often designed around 'how to do X and Y' rather than 'why should X or Y be true?'... The first question a good student should ask is "Why?". Your example is a good one to illustrate the error here, which is the failure to understand precisely what $sqrtx$ is defined as.
                    $endgroup$
                    – user21820
                    Mar 27 at 8:30










                  • $begingroup$
                    Well, my point of view is that, in one semester, I can't fix 12 years of students being taught math is only manipulation. (Not that every student has only had this experience, but enough will have had it that I need to make it a part of my planning.) So I do my best to get them to start thinking along that road at every opportunity.
                    $endgroup$
                    – kcrisman
                    Mar 27 at 13:19










                  • $begingroup$
                    Yup I understand that. =)
                    $endgroup$
                    – user21820
                    Mar 27 at 13:22















                  10












                  $begingroup$

                  This is a hard question, because students are so used to manipulation of this kind. I have found you are right that absolute values can cause the worst of these examples.



                  Here is an example I ran into recently, which I hope will help your thinking. Observe that there are two different limits here:
                  $$lim_xtopminfty fracxsqrtx^2+1 = pm 1$$



                  The "usual" way to proceed with these (informally, in many texts nowadays) is to divide numerator and denominator by the highest power, so:



                  $$lim_xtopminfty fracxcdot 1/xsqrt(x^2+1)cdot 1/x^2 = lim_xtopminfty frac1sqrt1+1/x^2=1$$



                  But of course bringing the $1/x$ inside the root like that is the same invalid manipulation you are mentioning.



                  In this case, we actually talked through it at an even more naive level, not more rigorous! Namely, as $xto -infty$, the numerator is negative and the denominator is positive. So the overall answer must be negative, no matter what the manipulation says. (You can graph it for them too.)



                  So in your case, I would go more naive as well. Do a very rough sketch of $arcsin(1/x)$ (you can basically do this by drawing $-arcsin(x)$ and then "flipping over $x=1$ to infinity"), and then ask them whether this function is increasing or decreasing. When $x<-1$ it should be increasing (in fact, it should be increasing on the whole domain), so its derivative should be positive (by whichever numbering of the fundamental theorems of calculus you like). But $frac1xsqrtx^2-1$ is definitely negative there.



                  enter image description here



                  Now you can explain why you are picky, instead of just being picky because of some "dumb" $|x|$ thing students may find to be a little too abstract.




                  Another answer brings up the question of whether this is a good question at all. But I think it is reasonable. What you may want to do, though, is find a way to discuss this "naturally", i.e. using the disconnect between what people write and then if they see what seems to be a "wrong" answer in the back of a book or something. Taking it as a first example in class probably will not register with them unless they are quite good at the concepts of calculus (not just mechanics), whereas pointing out why it is wrong/right should be better. (On another note, presumably there are branch cut issues here as well but presumably your class isn't ready for that!)






                  share|improve this answer









                  $endgroup$












                  • $begingroup$
                    And perhaps, one should be asking why in the first place students are "so used to manipulation of this kind". In my experience, this is a product of bad mathematical pedagogy, because the syllabus is often designed around 'how to do X and Y' rather than 'why should X or Y be true?'... The first question a good student should ask is "Why?". Your example is a good one to illustrate the error here, which is the failure to understand precisely what $sqrtx$ is defined as.
                    $endgroup$
                    – user21820
                    Mar 27 at 8:30










                  • $begingroup$
                    Well, my point of view is that, in one semester, I can't fix 12 years of students being taught math is only manipulation. (Not that every student has only had this experience, but enough will have had it that I need to make it a part of my planning.) So I do my best to get them to start thinking along that road at every opportunity.
                    $endgroup$
                    – kcrisman
                    Mar 27 at 13:19










                  • $begingroup$
                    Yup I understand that. =)
                    $endgroup$
                    – user21820
                    Mar 27 at 13:22













                  10












                  10








                  10





                  $begingroup$

                  This is a hard question, because students are so used to manipulation of this kind. I have found you are right that absolute values can cause the worst of these examples.



                  Here is an example I ran into recently, which I hope will help your thinking. Observe that there are two different limits here:
                  $$lim_xtopminfty fracxsqrtx^2+1 = pm 1$$



                  The "usual" way to proceed with these (informally, in many texts nowadays) is to divide numerator and denominator by the highest power, so:



                  $$lim_xtopminfty fracxcdot 1/xsqrt(x^2+1)cdot 1/x^2 = lim_xtopminfty frac1sqrt1+1/x^2=1$$



                  But of course bringing the $1/x$ inside the root like that is the same invalid manipulation you are mentioning.



                  In this case, we actually talked through it at an even more naive level, not more rigorous! Namely, as $xto -infty$, the numerator is negative and the denominator is positive. So the overall answer must be negative, no matter what the manipulation says. (You can graph it for them too.)



                  So in your case, I would go more naive as well. Do a very rough sketch of $arcsin(1/x)$ (you can basically do this by drawing $-arcsin(x)$ and then "flipping over $x=1$ to infinity"), and then ask them whether this function is increasing or decreasing. When $x<-1$ it should be increasing (in fact, it should be increasing on the whole domain), so its derivative should be positive (by whichever numbering of the fundamental theorems of calculus you like). But $frac1xsqrtx^2-1$ is definitely negative there.



                  enter image description here



                  Now you can explain why you are picky, instead of just being picky because of some "dumb" $|x|$ thing students may find to be a little too abstract.




                  Another answer brings up the question of whether this is a good question at all. But I think it is reasonable. What you may want to do, though, is find a way to discuss this "naturally", i.e. using the disconnect between what people write and then if they see what seems to be a "wrong" answer in the back of a book or something. Taking it as a first example in class probably will not register with them unless they are quite good at the concepts of calculus (not just mechanics), whereas pointing out why it is wrong/right should be better. (On another note, presumably there are branch cut issues here as well but presumably your class isn't ready for that!)






                  share|improve this answer









                  $endgroup$



                  This is a hard question, because students are so used to manipulation of this kind. I have found you are right that absolute values can cause the worst of these examples.



                  Here is an example I ran into recently, which I hope will help your thinking. Observe that there are two different limits here:
                  $$lim_xtopminfty fracxsqrtx^2+1 = pm 1$$



                  The "usual" way to proceed with these (informally, in many texts nowadays) is to divide numerator and denominator by the highest power, so:



                  $$lim_xtopminfty fracxcdot 1/xsqrt(x^2+1)cdot 1/x^2 = lim_xtopminfty frac1sqrt1+1/x^2=1$$



                  But of course bringing the $1/x$ inside the root like that is the same invalid manipulation you are mentioning.



                  In this case, we actually talked through it at an even more naive level, not more rigorous! Namely, as $xto -infty$, the numerator is negative and the denominator is positive. So the overall answer must be negative, no matter what the manipulation says. (You can graph it for them too.)



                  So in your case, I would go more naive as well. Do a very rough sketch of $arcsin(1/x)$ (you can basically do this by drawing $-arcsin(x)$ and then "flipping over $x=1$ to infinity"), and then ask them whether this function is increasing or decreasing. When $x<-1$ it should be increasing (in fact, it should be increasing on the whole domain), so its derivative should be positive (by whichever numbering of the fundamental theorems of calculus you like). But $frac1xsqrtx^2-1$ is definitely negative there.



                  enter image description here



                  Now you can explain why you are picky, instead of just being picky because of some "dumb" $|x|$ thing students may find to be a little too abstract.




                  Another answer brings up the question of whether this is a good question at all. But I think it is reasonable. What you may want to do, though, is find a way to discuss this "naturally", i.e. using the disconnect between what people write and then if they see what seems to be a "wrong" answer in the back of a book or something. Taking it as a first example in class probably will not register with them unless they are quite good at the concepts of calculus (not just mechanics), whereas pointing out why it is wrong/right should be better. (On another note, presumably there are branch cut issues here as well but presumably your class isn't ready for that!)







                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered Mar 25 at 13:49









                  kcrismankcrisman

                  3,633732




                  3,633732











                  • $begingroup$
                    And perhaps, one should be asking why in the first place students are "so used to manipulation of this kind". In my experience, this is a product of bad mathematical pedagogy, because the syllabus is often designed around 'how to do X and Y' rather than 'why should X or Y be true?'... The first question a good student should ask is "Why?". Your example is a good one to illustrate the error here, which is the failure to understand precisely what $sqrtx$ is defined as.
                    $endgroup$
                    – user21820
                    Mar 27 at 8:30










                  • $begingroup$
                    Well, my point of view is that, in one semester, I can't fix 12 years of students being taught math is only manipulation. (Not that every student has only had this experience, but enough will have had it that I need to make it a part of my planning.) So I do my best to get them to start thinking along that road at every opportunity.
                    $endgroup$
                    – kcrisman
                    Mar 27 at 13:19










                  • $begingroup$
                    Yup I understand that. =)
                    $endgroup$
                    – user21820
                    Mar 27 at 13:22
















                  • $begingroup$
                    And perhaps, one should be asking why in the first place students are "so used to manipulation of this kind". In my experience, this is a product of bad mathematical pedagogy, because the syllabus is often designed around 'how to do X and Y' rather than 'why should X or Y be true?'... The first question a good student should ask is "Why?". Your example is a good one to illustrate the error here, which is the failure to understand precisely what $sqrtx$ is defined as.
                    $endgroup$
                    – user21820
                    Mar 27 at 8:30










                  • $begingroup$
                    Well, my point of view is that, in one semester, I can't fix 12 years of students being taught math is only manipulation. (Not that every student has only had this experience, but enough will have had it that I need to make it a part of my planning.) So I do my best to get them to start thinking along that road at every opportunity.
                    $endgroup$
                    – kcrisman
                    Mar 27 at 13:19










                  • $begingroup$
                    Yup I understand that. =)
                    $endgroup$
                    – user21820
                    Mar 27 at 13:22















                  $begingroup$
                  And perhaps, one should be asking why in the first place students are "so used to manipulation of this kind". In my experience, this is a product of bad mathematical pedagogy, because the syllabus is often designed around 'how to do X and Y' rather than 'why should X or Y be true?'... The first question a good student should ask is "Why?". Your example is a good one to illustrate the error here, which is the failure to understand precisely what $sqrtx$ is defined as.
                  $endgroup$
                  – user21820
                  Mar 27 at 8:30




                  $begingroup$
                  And perhaps, one should be asking why in the first place students are "so used to manipulation of this kind". In my experience, this is a product of bad mathematical pedagogy, because the syllabus is often designed around 'how to do X and Y' rather than 'why should X or Y be true?'... The first question a good student should ask is "Why?". Your example is a good one to illustrate the error here, which is the failure to understand precisely what $sqrtx$ is defined as.
                  $endgroup$
                  – user21820
                  Mar 27 at 8:30












                  $begingroup$
                  Well, my point of view is that, in one semester, I can't fix 12 years of students being taught math is only manipulation. (Not that every student has only had this experience, but enough will have had it that I need to make it a part of my planning.) So I do my best to get them to start thinking along that road at every opportunity.
                  $endgroup$
                  – kcrisman
                  Mar 27 at 13:19




                  $begingroup$
                  Well, my point of view is that, in one semester, I can't fix 12 years of students being taught math is only manipulation. (Not that every student has only had this experience, but enough will have had it that I need to make it a part of my planning.) So I do my best to get them to start thinking along that road at every opportunity.
                  $endgroup$
                  – kcrisman
                  Mar 27 at 13:19












                  $begingroup$
                  Yup I understand that. =)
                  $endgroup$
                  – user21820
                  Mar 27 at 13:22




                  $begingroup$
                  Yup I understand that. =)
                  $endgroup$
                  – user21820
                  Mar 27 at 13:22











                  9












                  $begingroup$

                  You are right to be concerned that the students are "missing something", but IMO the real problem here is that the question is completely artificial.



                  In any application of this type of integral, most likely $x$ will be known to be either positive or negative, but not both, and only one part of the "either-or" answer would apply. And there had better be a good reason why the rest of the problem needs $x$ to be negative, when it could have been replaced by $-x$ right from the start!



                  The same is true for the more common case of the indefinite integral of $1/x$ when $x < 0$, of course.






                  share|improve this answer









                  $endgroup$

















                    9












                    $begingroup$

                    You are right to be concerned that the students are "missing something", but IMO the real problem here is that the question is completely artificial.



                    In any application of this type of integral, most likely $x$ will be known to be either positive or negative, but not both, and only one part of the "either-or" answer would apply. And there had better be a good reason why the rest of the problem needs $x$ to be negative, when it could have been replaced by $-x$ right from the start!



                    The same is true for the more common case of the indefinite integral of $1/x$ when $x < 0$, of course.






                    share|improve this answer









                    $endgroup$















                      9












                      9








                      9





                      $begingroup$

                      You are right to be concerned that the students are "missing something", but IMO the real problem here is that the question is completely artificial.



                      In any application of this type of integral, most likely $x$ will be known to be either positive or negative, but not both, and only one part of the "either-or" answer would apply. And there had better be a good reason why the rest of the problem needs $x$ to be negative, when it could have been replaced by $-x$ right from the start!



                      The same is true for the more common case of the indefinite integral of $1/x$ when $x < 0$, of course.






                      share|improve this answer









                      $endgroup$



                      You are right to be concerned that the students are "missing something", but IMO the real problem here is that the question is completely artificial.



                      In any application of this type of integral, most likely $x$ will be known to be either positive or negative, but not both, and only one part of the "either-or" answer would apply. And there had better be a good reason why the rest of the problem needs $x$ to be negative, when it could have been replaced by $-x$ right from the start!



                      The same is true for the more common case of the indefinite integral of $1/x$ when $x < 0$, of course.







                      share|improve this answer












                      share|improve this answer



                      share|improve this answer










                      answered Mar 25 at 18:47









                      alephzeroalephzero

                      32113




                      32113





















                          6












                          $begingroup$

                          To me, the key point here is that the integral runs over a singularity. If you naively calculates a definite form that runs over the singularity you get the wrong answer. This is something I have done enough so that I have taught myself to be careful in this case.



                          I am more a physicist than a mathematician, so what I care about is the connection to a practical situation, rather than the formal manipulation of symbols. If you or the students are of a similar inclination, then the presence of the singularity is what tips you off.



                          As you know, different students respond well to different approaches, so I mention this one so that you might add it to your arsenal.






                          share|improve this answer









                          $endgroup$

















                            6












                            $begingroup$

                            To me, the key point here is that the integral runs over a singularity. If you naively calculates a definite form that runs over the singularity you get the wrong answer. This is something I have done enough so that I have taught myself to be careful in this case.



                            I am more a physicist than a mathematician, so what I care about is the connection to a practical situation, rather than the formal manipulation of symbols. If you or the students are of a similar inclination, then the presence of the singularity is what tips you off.



                            As you know, different students respond well to different approaches, so I mention this one so that you might add it to your arsenal.






                            share|improve this answer









                            $endgroup$















                              6












                              6








                              6





                              $begingroup$

                              To me, the key point here is that the integral runs over a singularity. If you naively calculates a definite form that runs over the singularity you get the wrong answer. This is something I have done enough so that I have taught myself to be careful in this case.



                              I am more a physicist than a mathematician, so what I care about is the connection to a practical situation, rather than the formal manipulation of symbols. If you or the students are of a similar inclination, then the presence of the singularity is what tips you off.



                              As you know, different students respond well to different approaches, so I mention this one so that you might add it to your arsenal.






                              share|improve this answer









                              $endgroup$



                              To me, the key point here is that the integral runs over a singularity. If you naively calculates a definite form that runs over the singularity you get the wrong answer. This is something I have done enough so that I have taught myself to be careful in this case.



                              I am more a physicist than a mathematician, so what I care about is the connection to a practical situation, rather than the formal manipulation of symbols. If you or the students are of a similar inclination, then the presence of the singularity is what tips you off.



                              As you know, different students respond well to different approaches, so I mention this one so that you might add it to your arsenal.







                              share|improve this answer












                              share|improve this answer



                              share|improve this answer










                              answered Mar 25 at 22:10









                              AndrewAndrew

                              611




                              611





















                                  1












                                  $begingroup$

                                  It is interesting to note that if we instead write the antiderivative as $I=-arctanleft(frac1sqrtx^2-1right)$, then this form is valid for both $x<1$ and $x>1$. In other words, using this arctan representation, we avoid a need for a piecewise representation for the antiderivative.



                                  This problem is well suited for formative assessment. I would give full credit for an answer of the form $I=-arcsinleft(frac1xright)$, and then use this as a launching point for a discussion of these more involved issues that you have uncovered, depending on the mathematical maturity of the student.






                                  share|improve this answer









                                  $endgroup$

















                                    1












                                    $begingroup$

                                    It is interesting to note that if we instead write the antiderivative as $I=-arctanleft(frac1sqrtx^2-1right)$, then this form is valid for both $x<1$ and $x>1$. In other words, using this arctan representation, we avoid a need for a piecewise representation for the antiderivative.



                                    This problem is well suited for formative assessment. I would give full credit for an answer of the form $I=-arcsinleft(frac1xright)$, and then use this as a launching point for a discussion of these more involved issues that you have uncovered, depending on the mathematical maturity of the student.






                                    share|improve this answer









                                    $endgroup$















                                      1












                                      1








                                      1





                                      $begingroup$

                                      It is interesting to note that if we instead write the antiderivative as $I=-arctanleft(frac1sqrtx^2-1right)$, then this form is valid for both $x<1$ and $x>1$. In other words, using this arctan representation, we avoid a need for a piecewise representation for the antiderivative.



                                      This problem is well suited for formative assessment. I would give full credit for an answer of the form $I=-arcsinleft(frac1xright)$, and then use this as a launching point for a discussion of these more involved issues that you have uncovered, depending on the mathematical maturity of the student.






                                      share|improve this answer









                                      $endgroup$



                                      It is interesting to note that if we instead write the antiderivative as $I=-arctanleft(frac1sqrtx^2-1right)$, then this form is valid for both $x<1$ and $x>1$. In other words, using this arctan representation, we avoid a need for a piecewise representation for the antiderivative.



                                      This problem is well suited for formative assessment. I would give full credit for an answer of the form $I=-arcsinleft(frac1xright)$, and then use this as a launching point for a discussion of these more involved issues that you have uncovered, depending on the mathematical maturity of the student.







                                      share|improve this answer












                                      share|improve this answer



                                      share|improve this answer










                                      answered Mar 26 at 16:50









                                      user52817user52817

                                      3,359618




                                      3,359618



























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