$$ \lim\_{n→\infty} \frac1 n \left(\left(a + \frac 1 n\right)^2 + \left(a + \frac 2 n\right)^2 + ... + \left(a + \frac{n-1}{n}\right)^2\right)$$ 
$$ \text{hint: }\ 1^2 + 2^2 + ... + n^2 = \frac{n(n+1)(2n+1)}{6} $$

I can't figure out how to find this problem's limit. Does anybody have ideas?