Question

cos3xcosx+sin3xsin3=-12

Answer 1

cos3xcosx+sin3xsinx=-1/2
cos(3x-x) = - 1/2
cos2x= -1/2
2x= (2k+1)·π +/-π/3    ;  k∈Z
x= (2k+1)·π/2 +/-π/6