n=1:&
Assume+ ... + = for n=k
Let n=k+1:
+ ... + +
// Apply hypothesis to all but the last term
=+
// combine
=
// Here you can attempt to factor, or, you can see if (1/3)(k+1)(4(k+1)^2-1)
// expands to the same expression... I will do the latter approach...
=
=
=
=
They match, the hypothesis is also true for n=k+1, DONE.