Posted by Mitchel on .
Verify the identity.
cos 4x + cos 2x = 2  2 sin^2(2x)  2 sin^2 x

Trig 
Reiny,
using the identity
cos 2A = cos^2 A  sin^2 A = 1  2sin^2 x = 2cos^2 x 1
LS = cos 4x + cos 2x
= cos^2 2x  sin^2 2x + cos 2x
= (1  sin^2 (2x) )  sin^2 (2x) + (1  2sin^2 x)
= 1  2sin^2 (2x) + 1  2sin^2 x
= 2  2sin^2 (2x)  2sin^2 x
= RS