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 = 60

x = 60/2 = 30