Cho hàm số \(y=\cos 3x.\sin 2x\). Tính \(y'\left( \frac{\pi }{3} \right)\) bằng:
A. \(y'\left( \frac{\pi }{3} \right)=-1\)
B. \(y'\left( \frac{\pi }{3} \right)=\frac{1}{2}\)
C. \(y'\left( \frac{\pi }{3} \right)=-\frac{1}{2}\)
D. \(y'\left( \frac{\pi }{3} \right)=1\)