$15a^5+6a^3-21a^2$

$\left(\frac{2}{1-sin\left(x\right)}\right)+\left(\frac{2}{1+sin\left(x\right)}\right)$

$\frac{cos^4x-cos^2x}{sec\:x}$

$3h^2\:.2h^2$

$\sin^2x+9=10-\cos^2x$

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

$\lim_{x\to\infty}\left(ln\left(8x^3-5\right)-ln\left(2x^3+6\right)\right)$