cos9x-cos7x+cos3x-cosx=0

cos (9x) -cos (7x)+cos (3x) — cos (x)=0 (cos9x-cosx)=(cos 7x — cos 3x) -2sin[ (9x+x) /2]*sin[ (9x-x) /2]=-2sin[ (7x+3x) /2]*sin[ (7x-3x) /2] sin 5x*sin4x=sin5x*sin2x sin 4x=sin 2x 2sin2x*cos2x=sin2x 2cos2x=1 cos2x=1/2 2x=п/3=60x=60/2=30