$\frac{dy}{dx}=\frac{6x-x^3}{2y}$

$\left(-3+3u\right)+\left(7-2u\right)$

$7x-21$

$\left(2xy-\tan\left(y\right)\right)dx+\left(x^2-x\sec^2\left(y\right)\right)dy=0$

$cosa^2\:-\:2cosa\cdot cosb\:+\:cosb^2\:+\:sina^2\:-\:2sina\cdot sinb\:+\:sinb^2$

$\lim_{x\to\frac{\pi}{2}}\left(\frac{1+\sin\left(3x\right)}{\left(x-\frac{\pi}{2}\right)^2}\right)$

$tan\:cot\:=1$