26. limn→∞∑k=1nk2+2knk3+3k2n+n3\displaystyle\lim_{n\to\infty}\sum_{k=1}^{n}\frac{k^2+2kn}{k^3+3k^2n+n^3}n→∞limk=1∑nk3+3k2n+n3k2+2kn의 값은? [3점]