Sin^2 (3x) - cos^2 (3x) = 1

sin^2 (3x) - cos^2 (3x) = 1

косинус двойного аргумента

cos2a = cos^2 (a) - sin^2 (a)

- (cos^2 (3x) - sin^2 (3x)) = 1

- cos (2*3x) = 1

- cos6x = 1

cos6x = - 1

6x = pi + 2pik / : 6

x = pi/6 + pik/3, k ∈ Z