Limits Continuity And Differentiability

\(a \in R\) (the set of all real numbers), a \(\ne\) \(-\)1,

\(\mathop {\lim }\limits_{n \to \infty } {{({1^a} + {2^a} + ... + {n^a})} \over {{{(n + 1)}^{a - 1}}[(na + 1) + (na + 2) + ... + (na + n)]}} = {1 \over {60}}\), Then a = ?