Question

Prove the identity: 2cos^4(x)-cos^2(x)-2sin^2(x)cos^2(x)+sin^2(x)=cos^2(2x)

Answer 1

Cos^2(x)(2Cos^2(x)-1)-sin^2(x)(2Cos^2(x)-1)

Now take (2Cos^2(x)-1) again

(2Cos^2(x)-1)(Cos^2(x)-Sin^2(x))
Cos(2x)*(Cos(2x)) = Cos^2(2x)

I prefer the classical way of using paper.

Answer 2

Thanks!! :D

Answer 3

You are welcome.