#### Answer

$\frac{dy}{dx} = 20cos^{-5} x \times sin x $

#### Work Step by Step

$y = 5u^{-4}$
where,
$u = cos x$
Now,
$\frac{dy}{du} = -20u^{-5}$
and,
$\frac{du}{dx} = -sin x$
So,
$\frac{dy}{dx} = \frac{dy}{du} \times \frac{du}{dx}$
$\frac{dy}{dx} = 20u^{-5} \times sin x$
$\frac{dy}{dx} = 20cos^{-5} x \times sin x $