$\lim_{x\to1}\left(\frac{x^8-1}{x^{10}-1}\right)$

$\left(x^2y-3y\right)\frac{dy}{dx}+xy^3=0$

$\lim_{x\to-5}\left(\frac{x^2+10x+25}{-45-4x+x^2}\right)$

$\left(-5+a^2\right)\left(5+a^2\right)$

$x\frac{dy}{dx}-2y=2x$

$f.\:\left(2m-3n\right)\left(\:2m+3n\right)$

$4+2+-19$