$$13\cdot13$$

$$3y\:\cdot\:4y$$

$$\lim_{x\to0}\left(\frac{e^x-1}{e^{2x-1}}\right)$$

$$\frac{dy}{dx}=\frac{8x}{7y}$$

$$4\left(x+y\right)^2-28\left(x+y\right)\left(x-y\right)+49\left(x-y\right)$$

$$3a+18-12a+15a$$

$$4\cdot140$$