I made the denominator larger (n > 2 eventually) just to divide out the 5n. It works without it but it doesn't look as good.
And how is it within the boundary, 3n^2 - 3n + 1 = 1 for n=1, and for n > 1, 3n^2 - 3n + 1 > 1. If it mattered I could change it to (n-2)^3 or (n-3)^3 or something. I guess there'd be problems when n = 1 then.