Let $z \in D$. Show that the function $\Phi : t \mapsto L(tz)$ is differentiable on an open interval including $[-1,1]$ and give a simple expression for its derivative on $[-1,1]$.
Let $z \in D$. Show that the function $\Phi : t \mapsto L(tz)$ is differentiable on an open interval including $[-1,1]$ and give a simple expression for its derivative on $[-1,1]$.