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I am teaching Calc 1 right now and I want to give my students more interesting examples of integrals. By interesting, I mean ones that are challenging, not as straightforward (though not extremely challenging like Putnam problems or anything). For example, they have to do a $u$-substitution, but what to pick for $u$ isn't as easy to figure out as it is usually. Or, several options for $u$ work so maybe they can pick one that works but they learn that there's not just one way to do everything.

So far we have covered trig functions, logarithmic functions, and exponential functions, but not inverse trig functions (though we will get to this soon so those would be fine too). We have covered $u$-substitution. Thinks like integration by parts, trig substitution, and partial fractions and all that are covered in Calc 2 where I teach. So, I really don't care much about those right now. I welcome integrals over those topics as answers, as they may be useful to others looking at this question, but I am hoping for integrals that are of interest to my students this semester.

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What sort of institution and what sort of students? If they're students who are determined to work hard and get an "A" not because they want to understand math but because it will impress their future employers, then don't bother. –  Michael Hardy Nov 9 '12 at 0:13
    
What do your students want to learn about Calculus? What sorts of choices with respect to Calculus do you allow them to make? –  Doug Spoonwood Nov 9 '12 at 0:25
    
Maybe try a different coordinate system (polar, or something arbitrary). –  kineticfocus Nov 9 '12 at 3:11
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@MichaelHardy They're mostly engineering students. This is a class of 35 students. Surely a few can be interested in something like this. I will certainly not not bother. –  Graphth Nov 9 '12 at 13:12
    
@Doug I don't know what you mean by choices with respect to calculus. –  Graphth Nov 9 '12 at 13:13
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13 Answers 13

You might consider the old warhorse $$ \int \sec x\ dx $$ It's very common in calculus texts to resort to the trick of multiplying and dividing by $(\sec x + \tan x)$, upon doing which the answer jumps right out with a bit of simplification. Any reasonable student, though, might complain about this "rabbit out of the hat approach," asking, "How on earth could you expect me to come up with this idea?" All this approach does is impress the student with the author's cleverness while at the same time making them feel stupid. Here's an alternative approach that involves a different, and perhaps more accessible, kind of cleverness.

$$ \begin{align} \int\sec x\ dx&=\int\frac{1}{\cos x}\ dx=\int \frac{\cos x}{\cos^2 x}\ dx = \int \frac{\cos x}{1-\sin^2 x}\ dx\\ &= \int \cos x\left(\frac{1}{(1-\sin x)(1+\sin x)}\right)\ dx\\ \end{align} $$ Continue with partial fractions: $$ \begin{align} &=\int \frac{\cos x}{2}\left(\frac{1}{1-\sin x}+\frac{1}{1+\sin x}\right)\ dx\\ &= \frac{1}{2}\int\frac{\cos x}{1-\sin x}\ dx+\frac{1}{2}\int \frac{\cos x}{1+\sin x}\ dx\\ \end{align} $$ and now two simple substitutions and a bit of algebra gives the result. Occasionally, after giving this version I'll give the textbook version as an exercise, where it properly belongs.

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Good point on the possible reaction of the students to multiplying and dividing by $\sec x + \tan x$. –  Graphth Nov 9 '12 at 16:27
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See also "Ways to evaluate $\int \sec \theta \, d \theta$" –  Mike Spivey Dec 9 '12 at 21:26
    
want to do +2, but not allowed:) –  Yola Apr 6 at 10:08
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One pair of integrals they might find interesting is $$\int_0^{\pi/2} \cos^2 x \, dx \textrm{ and } \int_0^{\pi/2} \sin^2 x \, dx.$$

These integrals can be evaluated two different ways.

  1. Use double angle formulas to find the antiderivatives.

  2. Intuitively, the integrals should be the same, because they're the same function only flipped around. More formally, your students can check that if you make the substitution $u=\frac{\pi}{2}-x$ it turns one integral into the other. But their sum is $\int_0^{\pi/2} \sin^2 x + \cos^2 x \, dx=\int_0^{\pi/2} 1 \, dx$.

By the same trick, you can have your students integrate $$\int_0^{\pi/2} \frac{\sin^3 x}{\sin^3 x + \cos^3 x} dx$$

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I remember spending a lot of time trying to crack $$\int \frac{1}{\sqrt{x}+\sqrt[3]{x}}\,dx$$ back in the day, which became much simpler when I found out one could just let $u^6=x$. For your Calc 2 class, I've always been fond of $\int \sqrt{\tan{x}}\,dx$. It uses almost the whole cornucopia of tricks (substitution, completing the square, partial fractions).

Also, sometimes integrals which one might normally approach with trig substitution are much quicker if one knows about explicit formulae for inverse hyperbolic trig functions. For example, $$ \int\frac{1}{\sqrt{x^2+1}}\,dx$$ can be done with a trig substitution, or by noticing this is $\mathrm{arsinh}(x)+c$. In any case, you get $\log{(x+\sqrt{1+x^2}})+ c$, but it just depends whether you'd rather memorize the inverse trig formulae or do the trig subs.

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You taught me a new word, "cornucopia"! –  Sawarnik Mar 27 at 8:38
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Some of my favorite tricks:

Adding zero \begin{eqnarray*} \int \frac{dx}{1+e^x} &=& \int \frac{du}{u(u+1)} \hspace{10ex} (u = e^x) \\ &=& \int du \frac{1+u-u}{u(1+u)} \\ &=& \int \frac{du}{u} - \int \frac{du}{1+u} \\ &=& \ldots \end{eqnarray*}

Multiplying by one \begin{eqnarray*} \int \frac{dx}{1+e^x} &=& \int dx \frac{e^{-x}}{1+e^{-x}} \\ &=& -\int \frac{du}{1+u} \hspace{10ex} (u = e^{-x}) \\ &=& \ldots %&=& -\log(1+e^{-x}) + C \end{eqnarray*}

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This is exactly what I'm looking for! –  Graphth Nov 9 '12 at 13:10
    
@Graphth: I thought you and your students might like it. Cheers! –  user26872 Nov 9 '12 at 22:06
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One of my favorite tricks as well ! –  mick Nov 21 '12 at 21:33
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It's nothing fancy, but I think they are a must show to student to make them see past what they learn.

First one would be after you show them the change of variable technique. At first, student only want to seek what $u$ they can set to cancel something. Have them integrate something of the form $$ \int x\sqrt{x+1} dx. $$ Letting $u=x+1$ obviously leads to $$ \int (u-1)\sqrt{u} du $$ but most student won't know what to do with the $x$ infront of the initial square root.

Another one is right after you show them integration by part. Have them integrate $\ln(x).$

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Ah, but this is the kind of question that I'm looking for. I've seen this type of problem before, it's obvious to me. I can't remember if my students have seen one like this or not, but this is one of those where it's not as obvious. The derivative of $u$ isn't sitting there like it normally is. And, this one is simple enough that all my students should get it. Other great examples here that are more complicated might be nice for only a small part of the class. Thanks! –  Graphth Nov 9 '12 at 13:01
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The following example is interesting because there are several options for a substitution. This was on a test I was grading and to me one was obvious and it never occurred to me to do any other substitution. But, the students combined used several choices and to my surprise many worked.

$$\int \sec^8 x \tan x \,dx$$

To me, the obvious choice is $u = \sec x$, $du = \sec x \tan x \,dx$ which leads to

$$\int u^7 \,du = \frac{\sec^8 x}{8} + C$$

But, to my surprise, you can pick other powers of $\sec x$. If $u = \sec^n x$, where $n$ is a positive integer, then $du = n \sec^n x \tan x \,dx$, so $n = 1, 2, 4, 8$ all work. For example, if $u = \sec^4 x$, then $du = 4 \sec^4 x \tan x$ and thus we have

$$\frac{1}{4} \int u \,du = \frac{1}{4} \frac{(\sec^4 x)^2}{2} + C$$

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You might like $$ \int \dfrac{dx}{\sqrt{e^{2x}+1}}$$ which, a bit less than obviously, yields to the substitution $u = \sqrt{e^{2x}+1}$.

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I do like it.${}$ –  Graphth Nov 9 '12 at 15:20
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I think it is great to find a closed form formula for $$\Gamma(n,k)=\int_0^1 (-\log x)^{k-1}x^{n-1}dx$$

using integration by parts, where $n,k\in\Bbb N$.

One can prove that

$$\Gamma(n,k)=\int_0^1 (-\log x)^{k-1}x^{n-1}dx=\frac{(k-1)!}{n^{k}}$$

Informally, then, we have that

$$\begin{align} \sum\limits_{n = 1}^\infty {\Gamma (n,k)} &= \sum\limits_{n = 1}^\infty {\int_0^1 {{{( - \log x)}^{k - 1}}} {x^{n - 1}}dx} \cr &= \int_0^1 {\frac{{{{( - \log x)}^{k - 1}}}}{{1 - x}}dx} \cr &= \left( {k - 1} \right)!\sum\limits_{n = 1}^\infty {\frac{1}{{{n^k}}}} \cr &= \Gamma \left( k \right)\zeta \left( k \right) \end{align} $$

And maybe you can also introduce the Gamma function for integers as $$\Gamma \left( k \right) = \int_0^1 {{{( - \log x)}^{k - 1}}dx} $$

which can also be computed by integrations by parts and shown to be $=(k-1)!$.

Another nice which needs a tad more work, but which is pretty satisfying once is done is $$\int_0^{\pi /2}\sin^m\theta\cos^n\theta d \theta=\begin{cases} \frac{(m-1)!!(n-1)!!}{(m+n)!!} \text{ if any exponent is odd}\cr \frac{(m-1)!!(n-1)!!}{(m+n)!!}\frac{\pi} 2 \text{ both even exponents} \end{cases}$$

which can be used to prove Wallis's product.

You can see the details here.

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Rick Decker's answer mentioned the "old warhorse": the integral of the secant function. So I'll take that one a bit further:

  • There's a Wikipedia article about it: http://en.wikipedia.org/wiki/Integral_of_secant
  • It was tabulated by numerical methods in 1599.
  • It was first done in 1599 for the purposes of cartography: making maps of large portions of the world.
  • The closed form was conjectured in the 1640s. The conjecture became well known. Isaac Newton mentioned it.
  • It was the first integral ever done by partial fractions (see Rick Decker's answer).
  • The problem was solved in the 1660s.

Here's another entertaining integral: $$ \int_0^1 \frac{x^4(1-x)^4}{1+x^2}\,dx = \frac{22}{7} - \pi. $$ This integral also has its own Wikipedia article: http://en.wikipedia.org/wiki/Proof_that_22/7_exceeds_%CF%80


You might also consider whether some of the students will be entertained by the Weierstrass substitution: http://en.wikipedia.org/wiki/Weierstrass_substitution


This one uses integration by parts, so maybe you don't want to use it at this point: http://en.wikipedia.org/wiki/Integral_of_secant_cubed

This one is used in finding the arc length of the parabola and the Archimedean spiral, and the surface area of the helicoid.


(I'm the initial author of all of these articles. For the first, I relied heavily on an expository paper of V. Frederick Rickey and Philip M. Tuchinsky" "An Application of Geography to Mathematics: History of the Integral of the Secant", Mathematics Magazine, volume 53, number 3, May 1980, pages 162–166.)

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I think, in the same nature as Peter Tamaroff's answer, that it is always interesting to learn about the Gamma function:

$$\Gamma(z) := \int_0^\infty e^{-t}t^{z-1}\,\text dt$$

Integration by parts yields

$$\Gamma(z+1) = \left[-e^{-t}t^{z}\right]_0^\infty+z\int_0^\infty e^{-t}t^{z-1}\,\text dt = z\Gamma(z)$$

and seeing that

$$\Gamma(1) = \int_0^\infty e^{-t}\, dt = \left[-e^{-t}\right]_0^\infty = 1$$

thus for integers $n$:

$$\Gamma(n+1)=n!$$

and we see that the Gamma function is in fact an extension to the factorials for all $z \in \mathbb C$.

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I remember having fun with integrating some step functions, for example:

$$\int_{0}^{2} \lfloor x \rfloor - 2 \lfloor \frac{x}{2} \rfloor \ dx.$$

My professor for calculus III liked to make us compute piecewise functions, so it would force us to use the Riemann sum definition of the integral.

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Check out this, this and this and an extended discussion of some useful tricks here

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+1 for changingvariables.pdf –  Yola Apr 6 at 10:01
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How about the integral of the Standard Normal Distribution? i.e.

$$ \frac{1}{\sqrt{2 \pi}} \int_{-\infty}^{\infty} e^{-\frac{x^2}{2}}dx = 1 $$

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This is a great integral to show; but not for his purpose without students being exposed to multi-d calculus or being given many hints. –  dr jimbob Nov 9 '12 at 4:18
    
@drjimbob Correct, I'm not going to show this one to my students. It would take too much background. I will benefit most from the answers that are at the level of Calc 1, but others can benefit from this answer, those students or teachers at a higher level. –  Graphth Nov 9 '12 at 13:11
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