Prove that $coRP \subseteq RP^{RP}$
Suppose $L \in \mathsf{coRP}$, so that $\overline{L} \in \mathsf{RP}$. Using an oracle to $\mathsf{RP}$ we can determine whether a given string $x$ is in $\overline{L}$, and so whether $x \in L$. This gives a $\mathsf{P}^{\mathsf{RP}}$ algorithm for $L$.