It looks like your equation (it's not an identity) is
2 cos(5x) cos(3x) + sin(x) = cos(8x)
Recall that
cos(x + y) = cos(x) cos(y) - sin(x) sin(y)
cos(x - y) = cos(x) cos(y) + sin(x) sin(y)
==> 2 cos(x) cos(y) = cos(x + y) + cos(x - y)
so that
2 cos(5x) cos(3x) = cos(8x) + cos(2x)
Then the equation simplifies to
cos(8x) + cos(2x) + sin(x) = cos(8x)
cos(2x) + sin(x) = 0
Also recall that
cos(2x) = 1 - 2 sin²(x)
so the equation is quadratic in sin(x) and can be factorized:
1 - 2 sin²(x) + sin(x) = 0
2 sin²(x) - sin(x) - 1 = 0
(2 sin(x) + 1) (sin(x) - 1) = 0
Solve for x :
2 sin(x) + 1 = 0 or sin(x) - 1 = 0
sin(x) = -1/2 or sin(x) = 1
[x = arcsin(-1/2) + 2nπ or x = π - arcsin(-1/2) + 2nπ] or x = arcsin(1) + 2nπ
(where n is any integer)
x = -π/6 + 2nπ or x = -5π/6 + 2nπ or x = π/2 + 2nπ