sinx * sin5x = cos4x
1/2(cos(x-5x) - cos(x+5x)) = cos4x
1/2( - cos4x - cos6x ) = cos4x
cos4x + cos6x + 2cos4x = 0
3cos4x + cos6x = 0
6cos(4x+6x)/2 cos(4x-6x)/2 = 0
6cos5x (-cosx)=0
6cos5x=0 или cosx = 0
5x = п/2 + пк, к ∈ z
x = п/10 + пк/5, к ∈ z
x = п/2, пк, к ∈ z