$20\cdot70$

$\lim_{x\to0}\left(\frac{3x^3-x}{x}\right)$

$\frac{1-\sec\left(x\right)}{1-\cos\left(x\right)}=-\sec\left(x\right)$

$\left(-3x^2y^3+5a^2b^4\right)\left(-3x^2y^3-5a^2b^4\right)$

$2xy-3x=0$

$\ln\left(a\right)=\ln\left(b\right)+xy$

$x^4+8x^3\cdot14x^2-18x-15$