Limits Continuity And Differentiability

\(\sum_\limits{r=1}^n(2 r-1)=x\) then, \(\lim _\limits{n \rightarrow \infty}\left[\frac{1^3}{x^2}+\frac{2^3}{x^2}+\frac{3^3}{x^2}+\ldots+\frac{n^3}{x^2}\right]=\)