$\frac{x^4}{81}+\frac{1}{2}+\frac{81}{16y^4}$

$20+90+bc$

$\int_{\frac{\pi}{2}}^{\pi}\left(\cos\left(nx\right)\right)dx$

$\ln\left(x-5\right)+\ln\left(x+4\right)=ln\left(x-21\right)$

$\frac{dy}{dx}=\frac{1}{2}x+1$

$\lim_{x\to\infty}\left(\frac{x^2}{x\sqrt{x^2-1}}\right)$

$-7x^2-18x-7y^2=7$