I'm trying to find the first derivative of the associated Legendre function at $x=1$. The form I have for the first derivative is divergent at x=1: \begin{equation} \frac{d P_l^m(x)}{d \theta}=\frac{l x P_l^m(x)-(l+m) P_{l-1}^m(x)}{\sqrt{1-x^2}} \end{equation} I know that for any order $l$, the first derivative at $x=1$ is zero for all degrees $m$ except for $m=1$ or $m=-1$ as shown here https://i.stack.imgur.com/rqT5y.png by @brad14 in this question derivative of normalized associated Legendre function at the limits of x = +/-1
but I couldn't derive it. Would anyone help me with that, please?