\(y=cos\left(2x+\dfrac{\Omega}{3}\right)-cos2x\)
\(=-2\cdot sin\left(\dfrac{2x+\dfrac{\Omega}{3}+2x}{2}\right)\cdot sin\left(\dfrac{2x+\dfrac{\Omega}{3}-2x}{2}\right)\)
\(=-2\cdot sin\left(2x+\dfrac{\Omega}{6}\right)\cdot sin\left(\dfrac{\Omega}{6}\right)\)
\(=-sin\left(2x+\dfrac{\Omega}{6}\right)\)
=>\(-1< =y< =1\)
=>Chọn C