How could I simplify the following equation (C1) to become the next equation (C2) knowing that $r \gg d$ ($d$ is significantly smaller than $r$)
$\begin{align*} C_1 &= \frac{\epsilon_0 \pi r^2}{d + r - \sqrt{r^2+d^2}}\\ C_2 &= \frac{\epsilon_0 \pi r^2}{d} \end{align*}$