# Math Help - Finding cos(2A) given sin(A)

1. ## Finding cos(2A) given sin(A)

If A is a positive acute angle and sin A = (square root) 5/3, what is cos 2A ?

2. Originally Posted by Fenix Flaust
If A is a positive acute angle and sin A = (square root) 5/3, what is cos 2A ?
$\cos (2A) = 1 - 2 \sin^2 A$.