Show that if $(G,\cdot)$ is a group of order 9, then it's Abelian.
How to show this when you are given Lagrange's theorem and class equation?
Well, Lagrange's theorem basically states the following: $| G|/| H| = [G:H]$ when $H$ is a subgroup of $G$.
I don't see how to use this theorem on my problem?
