I know that the equivalence of two context-free grammars is undecidable, but what about the equivalence of a regular grammar and a context-free grammar?
Is equivalence of a CFG and an RG undecidable?
computability;formal grammars;decision problem
It is undecidable whether for a given CFG $G$, $L(G)=\Delta^*$, the set of all strings (over the terminal alphabet of $G$). That answers your question, by chosing the most simple regular grammar. 