# Math Help - trig identities

1. ## trig identities

I'm not sure how to deal with exponent 3 and 4

1. cos3x = 4cos^3x-3cosx

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

2. 1) $\cos 3x=\cos(x+2x)=\cos x\cos 2x-\sin x\sin 2x$

and use $\cos 2x=2\cos^2x-1, \ \sin 2x=2\sin x\cos x, \ \sin^2x=1-\cos^2x$

2) $\cos^4x=(\cos^2x)^2$

and use two times the identity $\cos^2\alpha=\frac{1+\cos 2\alpha}{2}$