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I'm working on a calculus problem and I need a push in the right direction.

The problem is:

$$\int\frac{5\sin(x)}{1-5\cos(x)}dx=$$

I've played with it for hours but I can't figure out where to start. Thanks!

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At this point, you probably know relatively few rules of integration, including the power rule, exponent rule, log rule, and trig rules.. By using a u-substitution (like the one Tyler suggests), you should be able to arrive at one of these basic integrals. –  The Chaz 2.0 Jun 17 '11 at 2:47
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4 Answers 4

up vote 8 down vote accepted

Do a $u$-substitution with $u=1-5\cos(x)$.

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Thanks bunches! –  InBetween Jun 17 '11 at 3:06
    
@InBetween: No problem, glad to help! –  Zev Chonoles Jun 17 '11 at 3:08
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The problem with "let $u = 1 - 5 \cos x$" is that, for beginners, it comes out of left field. Experts – even not so experts – see that substitution immediately. My own suggestion to my students was that you should check any integral against 4 kinds of known integrals: the integrals of powers $u^n$, sine, exponential, and $\sec^2$ (you have to know this one). That short list stands for more things: for powers, the special case $n = -1$; along with sine, the cosine; along with $e^u$, be prepared to consider $a^u$; in addition to $\sec^2$, $\csc^2$.

Having that list in your head, before you rush to integration by parts or general substitution, ask: is there some substitution that turns this given integral into one of those four?

In this case, the integral is certainly not $\sin u\; du$, $e^u\; du$, or $\sec^2 u\; du$. Is there any chance that it's $u^n\; du$?

If it is, then $n = -1$. that is, ask what happens if the stuff in the denominator is $u$, i.e. what happens if you set $u = 1 - 5 \cos x$. Compute $du$ and see if it collapses to $u^n\; du$. (Of course, it does.)

So if the substitution isn't obvious, this checklist gives you a suggestion of what to try.

For more detail about this approach, you could look at this post on my blog:

http://rip94550.wordpress.com/2010/01/25/calculus-organizing-techniques-of-integration/

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HINT Try a u-substitution with $u = 1 - 5\cos{x}$

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What do you know (calculus-wise) about the relationship between cosine and sine?

The derivative of the cosine function is the opposite of the sine function ($\frac{d}{dx}\cos x=-\sin x$), which suggests a $u$-substitution with $u$ having to do with cosine and $du$ having to do with sine.

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+1 for making this interactive! –  The Chaz 2.0 Jun 17 '11 at 2:48
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@The Chaz: I kind of wanted to just post the visible part, but it didn't feel like enough of an answer, and we do have this lovely spoiler-hiding feature... –  Isaac Jun 17 '11 at 2:50
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It's very socratic :) –  The Chaz 2.0 Jun 17 '11 at 3:16
    
www.artofproblemsolving.com website has this feature (hidden hints or answers), but here I see it for the first time +1 –  Theta33 Jun 17 '11 at 7:57
    
Oh wow, that's fancy! Very interesting +1 –  Tyler Jun 17 '11 at 14:34
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