$\:\lim_{h\to0}\left(\frac{\left(\left(x+h\right)^2\:\sqrt{\cos\left(x+h\right)}\right)-\left(x^2\sqrt{\cos\left(x\right)}\right)}{h}\right)$

$3m+4n-6n$

$4\left(3x+2y-9\right)$

$a^2+b^2+7a^2+9b^2$

$\int\sec^4x\left(\sec^2x-1\right)\cos xdx$

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

$\lim_{x\to\infty}\left(\frac{5}{12x+3}\right)$