Hey! :o
Find the solution of the problem $$u_{tt}(x, t)-u_{xx}(x, t)=0, 0<x<\pi, t>0 \tag {*} \\ u(0, t)=0, t>0 \\ u_x(\pi ,t)=-u_{tt}(\pi ,t), t>0$$
I have done the following:
We are looking for solutions of the form $$u(x, t)=X(x) \cdot T(t)$$
$$u(0, t)=X(0) \cdot T(t)=0 \Rightarrow...