Pls: cos^2x - sin^2x = 1.

 cos^2x - sin^2x = 1.
cos^2 4x=1
4x=2piN
x=PiN/2

cos² x – sin² x=cos2x=-1/2
2x=±arccos(-½)+2πk,  k ε  Z
x=±2π/6+πk=±π/3+πk.