$$36x^2-y^2$$

$$\cot\left(x-\frac{3\pi}{2}\right)=-\tan\left(x\right)$$

$$x+3xy^3-6x-2x+8xy^3+y-2x^4y$$

$$100a^2+20a+1$$

$$5x^2+2x+8-5x^2+3x$$

$$-3x^2y+15x^2y-24x^2y$$