However, to **maximize $r$**, we must **minimize the denominator** $\sin(2\theta)$, but not go below 0. So we consider the **maximum of $r^2$** under the constraint that $\sin(2\theta) > 0$. The maximum occurs when $\sin(2\theta)$ is at its **minimum positive value**, but that would make $r^2$ large — but we must find the **actual maximum possible finite value**. - Crosslake