Answer
$\frac{dy}{dx} = 3tan^{2} x \times sec^{2} x$
Work Step by Step
$y = u^{3}$
where,
$u = tan x$
Now,
$\frac{dy}{du} = 3u^{2}$
and,
$\frac{du}{dx} =sec^{2} x$
So,
$\frac{dy}{dx} = \frac{dy}{du} \times \frac{du}{dx}$
$\frac{dy}{dx} = 3u^{2} \times sec^{2} x$
$\frac{dy}{dx} = 3tan^{2} x \times sec^{2} x$
