Lagrange's Theorem: |H| divides |G|.
If G is a finite group and H is a subgroup of G, then |H| divides |G|. Moreover, the number of distinct left (right) cosets of H in G is |G|/|H|.
I need the proof of the first Corollary: |a| divides |G|.
In a finite group, the order of each element of the group divides the order of the group.