# Math Help - Finding a derivative

1. ## Finding a derivative

If y = cosx^2 - sinx^2, then what does y' equal?

The answer is -2sin(2x), but how do you actually get this answer? I used the power and chain rules when solving it, but the answer I got isn't the same. Thanks.

2. Originally Posted by pocketasian
If y = cosx^2 - sinx^2, then what does y' equal?

The answer is -2sin(2x), but how do you actually get this answer? I used the power and chain rules when solving it, but the answer I got isn't the same. Thanks.
$\cos^2(x)-\sin^2(x)=\cos\left(2x\right)$.

3. Oh, that's right, that's an identity. Thank you very much!