$\lim_{x\to-1}\left(\frac{1-x^{4}}{x^{6}-1}\right)$

$\left(-23\right)\cdot\left(-5\right)+\left(-23\right)\cdot\left(+19\right)$

$\frac{\left(-x^2+3x^4+2x+3\right)}{\left(x^2+2x\right)}$

$x^2+y^2\:=2x-y$

$x^2+3x=18$

$\sec^2x\left(\sin x+\cos x\right)$

$\lim_{x\to o}\left(\frac{\arctanh\left(2x\right)}{\sinh\left(3x\right)}\right)$