$\lim_{x\to4}\left(\frac{x-4}{\sqrt{-2}}\right)$

$\frac{du}{dx}=u^2+\frac{4u}{x}+\frac{4}{x^2}$

$\frac{2^{x+1}}{2^x-1}$

$n^2\left(n^2+2n+1\right)$

$x+\frac{4}{x}=1$

$\frac{\tan\left(a\right)\cot\left(a\right)}{\sec\left(a\right)}$

$\lim_{x\to3}\left(x^2+x-12\right)\lim_{x\to3}\left(x-3\right)$