cos x cos 3x = sin 2x sin 6x

cos (x) cos (3x) = sin (2x) sin (6x)

(1.2) * [cos (2x) + cos (4x) ] = (1/2) * [cos (4x) - cos (8x) ]

cos (2x) + cos (8x) = 0

2cos (5x) * cos (3x) = 0

a) cos (5x) = 0 = > 5x=± (pi/2) + pi*n = > x=± (pi/10) + pi*n/5

б) cos (3x) = 0 = > 3x=± (pi/2) + pi*n = > x=± (pi/6) + pi*n/3