There are three actually quite simple steps to doing this:
(1) Firstly, you must work out what the identity element is: .
(2) Secondly, you must work out the inverse of , an arbitrary element from your group.
(3) Lastly, you need to verify the result.
Which bit are you struggling with?
Thanks Swlabr for outlining the steps
so for the last step if e and f for T_e,f are rational then that means H is a normal subgroup in G. I had thought that if a subgroup is normal then every right coset is equal to every left coset i.e Ha = aH. is this also true.??
The elements of are the elements of the group of the form where is a rational number. So basically you have to show that if then . To do this you need to show that where . It does not matter what happens to .