√2[cos(P/4+3x) - cos7x] = 0
разность косинусов
-√2*2sin[(P/4+3x + 7x)/2]sin[(P/4+3x - 7x)/2] = 0
sin(P/8 + 5x)sin(P/8 - 2x) = 0
sin(P/8 + 5x) = 0 sin(P/8 - 2x) = 0
P/8 + 5x = Pk P/8 - 2x = Pk
5x = -P/8 + Pk -2x = -P/8 + Pk
x = -P/40 + Pk/5 x = P/16 - Pk/2
