)1) sin 2 x * cos 8 x= -sin x * cos 11x

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