# Math Help - Compound Angles

1. ## Compound Angles

If cosA = 3/5, 0deg <A<90deg write sinA as a fraction.
Hence write cos2A as a fraction.

I've got the first part sinA as a fraction, 4/5 . The second part, cos2A as a fraction is getting me. Any help?

2. Originally Posted by zyx.gar
If cosA = 3/5, 0deg <A<90deg write sinA as a fraction.
Hence write cos2A as a fraction.

I've got the first part sinA as a fraction, 4/5 . The second part, cos2A as a fraction is getting me. Any help?
Hi zyx.gar,

Try using the double angle identity:

$\cos 2A = \cos^2A-\sin^2A$