f(x)=√3cos(3x-θ)-sin(3x-θ)
=2[(√3/2)*cos(3x-θ)-(1/2)*sin(3x-θ)]
=2[cos(π/6)*cos(3x-θ)-sin(π/6)*sin(3x-θ)]
=2cos(π/6+3x-θ)
如果不懂,祝学习愉快!