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

• May 21st 2009, 05:21 PM
Fenix Flaust
Finding cos(2A) given sin(A)
If A is a positive acute angle and sin A = (square root) 5/3, what is cos 2A ?
• May 29th 2009, 04:31 AM
mr fantastic
Quote:

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

$\cos (2A) = 1 - 2 \sin^2 A$.