Question 971855
double angle identity for cos (x+y)

=cosx*cosy - sinx*siny

=cosx^2 - sinx^2  since y=x

But cos^2 x + sin^2 x =1

Therefore, cos (2x)= 1-sin^2 x -sin^2 x substituting in two lines above.

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