Cos x + cos 4x +cos 7x=0 sin2xsin6x - cosx cos 3x

1)cos x + cos 4x +cos 7x=0
2cos4xcos3x+cos4x=0
cos4x(2cos3x+1)=0
cos4x=0⇒4x=π/2+πn⇒x=π/8+πn/4
cos3x=-1/2⇒3x=+-2π/3+2πn⇒x=+-2π/9+2πn /3
2)sin2xsin6x - cosx cos 3x =1/2cos4x-1/2cos8x-1/2cos2x-1/2cos4x=-1/2(cos8x+cos2x)=
=-1/2*2cos5xcos3x=-cos5xcos3x