$cos\left(270+x\right)=\:-\cos\left(x\right)$

$\lim_{x\to\infty}\left(\frac{-3x^4+4x^3+8x^2-8x}{4x^3-7x^2+4x-3}\right)$

$\frac{\left(cos2a\right)}{\left(sena\right)}+sena=coseca$

$4x^2y^6-9m^2$

$\left(1\right)^2-\left(-8xy\right)^2$

$\frac{d}{dx}arccos2x^2$

$\lim_{x\to0}\frac{10^x-1}{2^x-1}$