Sin and Cos Integrals

1. Feb 2, 2009

XJellieBX

1. The problem statement, all variables and given/known data
Compute $$\int^{\pi/2}_{0} \frac{sin^{2009}x}{sin^{2009}x + cos^{2009}x}$$

I used the identity $$cos^{2}= 1 - sin^{2}$$, but instead I set the exponent as 2009. And so I ended up with the answer being -1. I'm just wondering whether this is a legal solution or am I not allowed to do that. Thanks.

2. Feb 2, 2009

AssyriaQ

$$sin^{2}x + cos^{2}x = 1$$ is the so called Pythagorean trigonometric identity. It is not valid when replacing the exponent 2 by another number, i.e.,

$$sin^{n}x + cos^{n}x \neq 1$$ for $$n\neq 2$$.

3. Feb 2, 2009

XJellieBX

Thank you, I really needed that second opinion =)

4. Feb 3, 2009

Dick

Try the change of variables x -> pi/2-x to get a new integral. Then add it to the old integral.

5. Feb 3, 2009

XJellieBX

Thank you =) I found the answer.