Theorem 11.4 (Second Isomorphism Theorem)Let H be a subgroup of a group G (not necessarily normal in G)and N a normal subgroup of G. Then HN is a subgroup of G,HN is a normal subgroup of H,and H/H∩N≈HN/N. HN是子群？ H∩N是正规子群？ H/H∩N和HNN到底是什么样子的群？HN 是子群？ H∩N 是正规子群？ H/H∩N 和HN/N到底是什么样子的群？