$\lim_{x\to81}\left(\frac{\sqrt{82-x}-1}{9-\sqrt{x}}\right)$

$\int-0.015e^{-0.01x}dx$

$\frac{dy}{dx}y=\left(\frac{\left(x+1\right)\left(x-4\right)}{\left(x-1\right)\left(x+4\right)}\right)$

$\cos\left(-20\right)\cos\left(-50\right)-\sin\left(-20\right)\sin\left(-50\right)$

$\int_0^1t\:e^{-t}dx$

$\left(\frac{x+1}{x^2-4x}\right)$

$x^2-5x+6\ge x+5$