20.7. sin 3x + sin 5x = sin 4x
2sin 4x*cos x = sin 4x
1) sin 4x = 0; 4x1 = pi*k; x1 = pi/4*k
2) 2cos x = 1; cos x = 1/2
x2 = +-pi/3 + 2pi*n
20.8. cos x - cos 3x = sin 2x
-2sin 2x * sin (-x) = sin 2x
2sin 2x*sin x = sin 2x
1) sin 2x = 0; 2x = pi*k; x1 = pi/2*k
2) 2sin x = 1; sin x = 1/2; x2 = (-1)^n*pi/6 + pi*n