Решение: 2sin (5x) sin (2x) = cosx

Sina*sinb=1/2[cos (a-b) - cos (a+b) ]

2sin5x*sin2x=cos (3x) - cos (7x)

cos3x-cos7x=cosx

cos3x=cos7x+cosx

cos3x=2cos3xcos4x

cos3x (1+2cos4x) = 0

1. cos3x=0 3x=π/2+πn x=π/6+πn/3 n∈Z

2. 1+2cos4x=0

cos4x=-1/2

4x=+-2π/3+2πn x=+-π/6+πn/2 n∈Z