CLRS Rod Cutting Inductive proof
First, be more precise in your writing. What does $\max$ range over? What is $k$ in the inductive hypothesis and step? What is the actual statement in the induction hypothesis?

In the step, you want to establish that $r_n = \dots$ is optimal assuming all $r_k$ as computed by the proposed formula for $k \leq n$ are optimal, for some fixed $n \geq 3$.

One way you typically do this is by contradiction. Assume (in addition to IH) that $\max \dots$ were not optimal. Derive that the chosen $r_k$ is not optimal either, then, which is in contradiction with the IH.