Tính giới hạn sau:
1) \(\lim\limits_{n\rightarrow\infty}\dfrac{1}{n^3}\left(1+2^2+...+\left(n-1\right)^2\right)\)
2) \(\lim\limits_{n\rightarrow\infty}\dfrac{1}{n}[\left(x+\dfrac{a}{n}\right)+\left(x+\dfrac{2a}{n}\right)+...+\left(x+\dfrac{\left(n-1\right)a}{n}\right)]\)
3) \(\lim\limits_{n\rightarrow\infty}\dfrac{1^3+2^3+...+n^3}{n^4}\)