
Lagrange's Theorem
Let G be a group and H a subgroup of G. Define, for a,b elements of G, a is an equivalence relation to b if a^1b element of H. Prove that this defines an equivalence relation on G and show that [a] = aH = {ah,h is an element of H}. The sets aH are called left cosets of H in G

This is pretty straight forward, just check the three properties of an equivalence relation. Recall an equivalence relation is reflexive, symmetric, and transitive. Here's a start.
Let
If , then since is a subgroup, (symmetry)
Now you need only show:
(reflexivity)
and
If and , then (transitivity)
For the second part, show containment both ways, and . It can be done in one fell swoop with a short string of equivalences if implications in both directions seems redundant.

Thanks Lepton..
I am stil having trouble showing that [a] is a subset of aH and aH is a subset of [a]..Please can you take me through the steps??