Question

cos3xcosx-sin3xsinx=

Answer 1

cos3x*cosx -sin3x*sinx =  -1/2 ;
cos(3x+x) = -1/2 ;
cos4x = -1/2 ; 
4x = +,- 2π/3 +2πn ,n∈Z.
x = +,- π/6 +(π/2)*n ,n∈Z.