$\frac{\left(x^2+1\right)}{\left(3x-x^2\right)}$

$4-\left(-41\right)$

$4mn-5mn-9mn+8m^2n$

$\left(8^3x\sqrt{36-2^5+2^3+\sqrt{16+2}}\right)$

$\frac{1-\cos\left(-x\right)}{\sec\left(-x\right)-1}=\cos x$

$169x^4-13x^2y^2+\frac{y^2}{4}$

$\lim_{x\to\left(-\frac{\pi}{2}\right)}\left(\frac{2x\sec\left(x\right)+\pi\sec\left(x\right)}{2}\right)$