\(\rm \displaystyle\sum_{r=1}^n (2r-1)=x\) then \(\rm \displaystyle\lim_{n\rightarrow \infty} \left[\dfrac{1^3}{x^2} + \dfrac{2^3}{x^2} + \dfrac{3^3}{x^2} + ... + \dfrac{n^3}{x^2} \right]=\)