\(\lim\limits_{n \to ∞}\frac{1+2+3+.....+n}{n^2}\)
\(=\lim\limits_{n \to ∞}\frac{\frac{n(n+1)}{2}}{n^2}\)
\(=\frac{1}{2}\lim\limits_{n \to ∞}\frac{n^2+n}{n^2}\)
\(=\frac{1}{2}\lim\limits_{n \to ∞}(1+\frac{1}{n})\)
\(=\frac{1}{2}(1+0)\)
\(=\frac{1}{2}\times1\)
\(=\frac{1}{2}\)