$\int\left(\frac{5x}{x^2-81}\right)dx$

$\frac{t^{11}}{t^2}$

$2x^2-20x+25$

$\frac{tan\left(x\right)}{cot\left(x\right)}+\frac{tan\left(z\right)}{\cot\left(z\right)}$

$\left(x^2-9\right)\frac{dy}{dx}=-xy$

$\lim_{b\to\infty}\left(\frac{e^{-b}}{\left(1+e^{-b}\right)^2}\right)$

$e^{-x^{-2}}$