Hallo,
cos(3x)= 4 cos^3 x - 3 cos x
2 cos(x) * cos(3x) = 2 cos(x) *(4 cos^3 x - 3 cos x ) = 8 cos^4(x) -6 cos^2(x)
cos(2x)+cos(4x):
cos(2x)= 2 cos^2(x) -1
cos(4x)= 8cos^4 x - 8 cos^2 x + 1
Wenn Du das einsetzt:
8 cos^4(x) -6 cos^2(x) = 2 cos^2(x) -1 +8cos^4 x - 8 cos^2 x + 1
8 cos^4(x) -6 cos^2(x) = 8cos^4 x - 6 cos^2 x