$\lim_{x\to7}\left(\frac{7-x}{343-x^3}\right)$

$\left(1+\frac{1}{x}+\frac{1}{y}+\frac{1}{xy}\right)\frac{dy}{dx}=1$

$\sec^2x\cot^2x\:=\:\csc^2x$

$m^2-n^4$

$\tan^2\left(x\right)-\sec^2\left(x\right)=\cos^2\left(x\right)$

$\frac{d^2}{dx^2}\frac{4}{x+3}$

$5-6\left[2t-2\left(t+3\right)\right]=8-t$