$-18\:.\:-20$

$\int\left(\frac{\sqrt{4x^2+1}}{x^4}\right)dx$

$\lim_{x\to0}\left(\frac{\frac{\pi}{2}-x\left(\sin\left(x-\frac{3\pi}{2}\right)\right)}{\sec\left(x-\frac{\pi}{2}\right)-1}\right)$

$\frac{di}{dt}=-i\:+s$

$\frac{d}{dx}x^3-y^2=xy$

$\frac{cscx}{8}+\frac{cscx}{4}+\frac{cscx}{2}+cscx+ctgx$

$\frac{8x^2-13x-1}{x\left(x-1\right)\left(x+1\right)}$