sin2x * cos8x=1/2(sin10x+sin(-6x))=1/2(sin10x-sin6x)
-sinx * cos11x=-1/2(sin12x+sin(10x))=-1/2(sin12x-sin10x)
sin10x-sin6x=-sin12x+sin10x
sin12x-sin6x=0
2 sin3x cos9x=0
a) sin3x=0 , 3x=πn , x=πn/3 , n∈Z
b) cos9x=0 , 9x=π/2+πk , x=π/18+πk/9 , k∈Z
