$($Ex: If $z=f(x,y)$ has cts $2^{nd}-$order partial derivatives and $x=r^2+s^2,y=2rs.$

Find $(1)\frac{\text{d}\text{e}\text{l}\text{z}}{\text{d}\text{e}\text{l}\text{r}}$

$(2)\frac{de{l}^{2}z}{del{r}^2}$

H$.$W