# How do you find the derivative of (cosx)(sinx)?

Use $\sin \left(2 a\right) = 2 \sin A \cos A$ to rewrite.
$y = \cos x \sin x = \frac{1}{2} \sin \left(2 x\right)$
$y ' = \frac{1}{2} \left[\cos \left(2 x\right) \cdot \frac{d}{\mathrm{dx}} \left(2 x\right)\right] = \frac{1}{2} \cos \left(2 x\right) \cdot 2 = \cos \left(2 x\right)$