Evaluating Integral of a Trigonometric Function: Homework Solution

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Homework Statement


Evaluate the integral with respect to f of the following function
http://img233.imageshack.us/img233/5303/picture1ub7.png



The Attempt at a Solution



This if for one of my homeworks. I haven't taken a single math course in almost 2 years so I'm kind of rusty. I'm not sure how to evaluate this. I tried moving the 'a' outside the integral but that doesn't help. I believe there is a trig identity I'm supposed to be able to use. Any tips or hints are greatly appreciated.
 
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That is for the form \frac{a^2}{a^2+b^2f^2}= \frac{1}{1+ \left(\frac{bf}{a})^2}.
Does it help you to know that the derivative of arctan(x) is \frac{1}{x^2+ 1}?
 
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