cos x - sin 4x = cos 3x cos x - cos 3x - sin 4x =0 2sin2xsinx- 2sin2xcos2x =0 2sin2x(sinx-cos2x)=0 sin2x=0 2x=pik x=pik/2, k∈Z sinx-cos2x=0 2sinx^2x+sinx-1=0 sinx=t 2t^2+t-1=0 D=9 t=(-1+3)/4= 1/2 t=(-1-3)/4= -1 x= -pi/2+2pik, k∈Z x= pi/6+2pik, k∈Z x= 5pi/6+2pik, k∈Z
