Question

How do I rewrite cos^4 x so that it involves only the first power ?

Answer 1

1-cos^4x then again use cos^2x-1=cos2x relation

Answer 2

One way of doing this...

sin^2(x) = 1/2 (1-cos(2 x))

also

anything^4 = (anything^2)^2

Hope this helps. :)