Differentiating cosine

If y=a\cos bx, then \frac{dy}{dx}=-ab\sin ax

Find \frac{dy}{dx} of y=\frac13\cos x

• \frac{dy}{dx}=-\frac13\sin x

Find \frac{dy}{dx} of y=\cos2x

• \frac{dy}{dx}=-2\sin2x

Find \frac{dy}{dx} of y=-\cos7x

• \frac{dy}{dx}=7\sin7x

Find \frac{dy}{dx} of y=2\cos4x

• \frac{dy}{dx}=-8\sin4x