题目如下：
Suppose that (G,‧) is a group and G = <a>. Prove that G = <a^k> if and
only if gcd(k, |G|) = 1, where k is a positive integer.
请问有人会证吗囧/
这貌似是Lagrange定理的引理，可是我实在是不会证XD"
请大家帮帮忙了！