Math Help - verifying identities

1. verifying identities

hey
im new to the site but i have some ap calc summer work due tomorrow and i am STUCK with this trig identity. any ideas how to start it off?

(cos^4)x = 1/8 (3+4cos2x+cos4x)

2. Originally Posted by still_nights
hey
im new to the site but i have some ap calc summer work due tomorrow and i am STUCK with this trig identity. any ideas how to start it off?

(cos^4)x = 1/8 (3+4cos2x+cos4x)
$(\cos^2 x)^2 = \left( \frac{\cos (2x) + 1}{2}\right)^2$
using the well known double angle formula $\cos (2x) = 2 \cos^2 (x) - 1$.