Problem: If H and K are subgroups of G, then |K : H \cap K| \le |H v K : H | , where H v K = <H \cup K > denotes the join of H and K. Fact: If H and K are subgroups of G, then |K : H \cap K | \le |G : H|. Am I correct in thinking that the problem would follow trivially from preceding fact, since H and K are subgroups of H v K?