Calculate $\frac{d}{dx}\cos(u)$ for the following choices of $u(x)$ :
$u(x)= 3-x^2$, $\frac{d}{dx}\cos(u(x))=$

$u(x)= x^{-6}$, $\frac{d}{dx}\cos(u(x))=$

$u(x)= \tan(x)$, $\frac{d}{dx}\cos(u(x))=$