# Homework Help: Integral problem

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1. Mar 17, 2015

### PWiz

1. The problem statement, all variables and given/known data
I was trying to find a general expression for the arc length of sine, i.e. $\int \sqrt{1+cos^2 x} dx$, but got stuck. I made this problem up, and with some google searches realized that it uses something known as an elliptical integral. How do I go about using it here?

2. Relevant equations

Trig substitutions

3. The attempt at a solution
By letting $cosx=tanu$ and by differentiating arccos, I came down to the integral $\int \frac{-sec^3 u}{\sqrt{2-sec^2 u}}du$, which seems as unsolvable as the original question.

Last edited: Mar 17, 2015
2. Mar 17, 2015

### Ray Vickson

Your integral is non-elementary, so no matter how hard you look and how many millions of pages you allow yourself for writing out a finite formula, you won't be able to do it. That is a genuine theorem---that is, has been proved rigorously. However, you can do it in terms of the non-elementary elliptic functions.

Elliptic functions are widely tabulated and are implemented in many computer algebra systems (Maple, Mathematica, etc.). There are also numerous sources of computer codes for them in various computer languages. You can also look on-line for more details, giving things like series expansions, asymptotic expansions and the like. What you probably will not find is a hand-held calculator with an "elliptic function" button (but I could be wrong).

Another approach would be straight numerical integration, where the actual function details are more-or-less ignored.

3. Mar 17, 2015

### Dick

To express the result as an elliptic integral you just need the definition of an elliptic integral of the second kind. $E(x|m)=\int_0^x \sqrt{1-m \sin^2(\theta)} d\theta$. If you can express your integral in terms of that form (find m), then you can express the result in terms of $E$.

4. Mar 17, 2015

### PWiz

Alright, say it was a definite integral:
$$\int_0^4 \sqrt{1+cos^2x} dx$$
What should my procedure then be?

5. Mar 17, 2015

### Ray Vickson

Do you want a numerical answer? If so, either apply an elliptic function software package, or else use a numerical integration scheme such as Simpson's Rule.

6. Mar 17, 2015

### PWiz

So you mean there is no way to calculate the exact value of the finite integral as well? Will I always be forced to use numerical approximations with these kind of functions? If yes, then is there any quick way to realize that a function is non-integrable (and hence save myself a lot of effort)?

Last edited: Mar 17, 2015
7. Mar 17, 2015

### Dick

Use the identity $cos^2 x=1-sin^2 x$ to express it in the elliptic form. If you are expecting to get an 'exact value' out of this you going to be disappointed unless you consider something like $sin(1)$ an 'exact value'.

8. Mar 17, 2015

### PWiz

So the definite integral is simply represented by $E(4|cot^2x)$? And what do we do if the lower limit is non-zero?

9. Mar 17, 2015

### Dick

No, how did you get $cot^2 x$??

10. Mar 17, 2015

### PWiz

Sorry, I meant $-cot^2x$. Since $1-msin^2x=1+cos^2x$, $m=-cot^2x$ right?

11. Mar 17, 2015

### Dick

No, $m$ needs to be a constant! Start from $\sqrt{1+cos^2 x} = \sqrt{1+(1-sin^2 x)}=\sqrt{ 2-sin^2 x}$. Now you are getting close to the elliptic form, except that '2' needs to be a '1'. Do you see what to do?

12. Mar 17, 2015

### SteamKing

Staff Emeritus
There's no quick way to decide whether a given integral is non-elementary, other than to look in a table of integrals and see if it, or one of its forms, appears.

There are some famous integrals which don't have closed-form solutions, but which are studied because of their utility in physics, mechanics, and other sciences. Elliptic integrals are a broad category of such functions, and these arose out of attempts to find a formula for the arc length of an ellipse.

http://en.wikipedia.org/wiki/Jacobi_elliptic_functions

One other famous non-elementary integral which finds daily use is the normal distribution, which appears quite often in statistics.

http://en.wikipedia.org/wiki/Normal_distribution

Other non-elementary functions are the result of trying to solve certain classes of ordinary differential equations. Bessel functions are one such class of solutions:

http://en.wikipedia.org/wiki/Bessel_function

As you go thru your mathematics education, you'll find that there are quite a few integrals and solutions to ODEs which cannot be expressed in terms of elementary functions.

13. Mar 17, 2015

### Ray Vickson

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The (now) free article
http://www4.ncsu.edu/~singer/ma792Kdocs/rosenlicht.pdf
has the basic results and gives proofs. See also
http://www.rangevoting.org/MarchisottoZint.pdf

Just Google 'integration if finite terms' to see more articles.

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Note added in edit:
There is an article by Mathew Wiener that goes through all this in a fairly elementary way, taking the time to introduce the tools and prove results in about as simple a way as possible. It also lists a number of provably-non-elementary integrations, which should suit your purposes of knowing when to stop trying to integrate and when to use other methods (such as numerics). Here is the link:

http://www.math.niu.edu/~rusin/known-math/93_back/elementary.int [Broken]

I recommend you look at it.

Last edited by a moderator: May 7, 2017
14. Mar 18, 2015

### PWiz

@Dick Okay, if m is supposed to be a constant then will the integral resolve out to become $\sqrt{2} E(4|0.5)$?
@Ray Vickson Thanks for the articles, there is definitely some interesting information in them on which I'd like to spend some more time.
@SteamKing I'm familiar with none of those functions except the normal distribution.

15. Mar 18, 2015

### Dick

Yes!

16. Mar 20, 2015

### PWiz

And what if one of the limits isn't zero, like $\int_{-2}^4 \sqrt{1+cos^2x}dx$?

17. Mar 20, 2015

### Dick

I'm not going to tell you until you think about it.

18. Mar 20, 2015

### PWiz

I was thinking about splitting the problem into two : $-\int_0^{-2} \sqrt{1+cos^2x}dx$ + $\int_0^4 \sqrt{1+cos^2x}dx = \sqrt{2} (-E(-2|0.5)+E(4|0.5))$, but I still have my doubts.

19. Mar 23, 2015

### PWiz

Um so did I get it right?

20. Mar 24, 2015

### Ray Vickson

Look at areas under the curve $y = f(x) = \sqrt{1+\cos^2(x)}$. The quantity, $\int_{-2}^4 f(x) \, dx$, is the area $A(-2 \to +4)$ under $y = f(x)$ between $x = -2$ and $x = +4$. This is $A(-2 \to 0) + A(0 \to +4)$. Look at $f(x)$. For $a > 0$ can you see how to relate $A(-a \to 0)$ to values of $A(0,b)$ for some appropriate $b > 0$?

21. Mar 24, 2015

### PWiz

Yes, and after splitting the problem into those two parts (post 18) and switching the limits for the -ve case (by adding a -ve sign outside the integral) I came down to $\sqrt{2} (E(4|0.5)-E(-2|0.5))$. I can't simplify this any further, so have I done it correctly?

22. Mar 24, 2015

### Dick

Sure that's right. You can also check these sorts of thing on Wolfram. Ask it for the value of sqrt(2)*(EllipticE(4,1/2)-EllipticE(-2,1/2)). Compare it with the value it gives for the integral.

23. Mar 24, 2015

### PWiz

Alright, thanks. One final question: are these sort of problems encountered in math during the first two years of undergrad education?

24. Mar 24, 2015

### Dick

I don't think special functions like that are a routine part of the curriculum. On the other hand you might see them in a course with a title like 'Mathematical Physics'.

25. Mar 24, 2015

### Ray Vickson

OK, but if you notice that $f(x) = \sqrt{1+\cos^2(x)}$ is an even function (meaning that $f(-x) = f(x)$) you ought to be able to see that $A(-2 \to 0) = A(0 \to 2)$, so the answer is $E(4) + E(2)$. That would be convenient if you were using tables of $E$ (or software) that could only evaluate it for positive arguments.

That was why I asked you about the relationship between $A(-2 \to 0)$ and $A(0 \to 2)$, but you did not answer my question.