$\left(1-\sin\left(a\right)^2\right)\sec^4\left(a\right)$

$15x^3+20x^3-x^3$

$\left(0.5b\right)\left(3.5b-4\right)$

$\lim_{x\to1}\frac{x^5-x^8}{1-x^6}$

$\lim_{x\to\infty}y^n$

$\left(-432\right)-\left(1889\right)$

$\left(-3x^2+6xy^2\right)dx+\left(6x^2y+4y^3\right)dy=0$