$\log_{19}\left(21-2r\right)=\log_{19}\left(r^2+2r\right)$

$100-12+5-11+12-13+8-11+4-15+6-13-9+8$

$\int\:\frac{x^2}{\sqrt{81-x^2}}dx$

$2-0+90+51$

$\left(x^6+4\right)\left(x^6+8\right)$

$x-3y-2z=7$

$f\left(t\right)=\left(e^{\left(-t\right)}\:+e^{\left(t\right)}\right)^2$