# Compute $\lim\limits_{n \to \infty }\frac{981}{n+5}\sum_{i=1}^{n} \left (\frac{i^2}{n^2} \right)$

Compute the given limit $$\lim_{n \to \infty }\frac{981}{n+5}\sum_{i=1}^{n} \left (\frac{i^2}{n^2} \right)$$ The sum is:

Can someone please show me the steps to complete this problem? The answer I arrived at was 0 but the homework program is telling me that it's wrong.

Thank you.

• You should consider upvoting or accepting an answer (by clicking the $\checkmark$) if an answer helped you. – Eff Nov 26 '14 at 22:14

Firstly calculate the sum$$\sum_{i=1}^{n}\frac{i^2}{n^2}=\frac{1}{n^2}\sum_{i=1}^{n}i^2=\frac{1}{n^2}\frac{n(n+1)(2n+1)}{6}=\frac{\not n^2}{\not{n^2}}\frac{(1+\frac1n)(2n+1)}{6}$$ Therefore \begin{align*}\lim_{n \to \infty}\frac{981}{n+5}\frac{(1+\frac1n)(2n+1)}{6}&=\lim_{n\to \infty}\frac{\not n}{\not n}\frac{981(1+\frac1n)(2+\frac1n)}{6(1+\frac5n)}\overset{\frac1n\to 0}=\\\\&=\frac{981(1+0)\cdot(2+0)}{6(1+0)}=\frac{981\cdot2}{6}=\frac{981}{3}=327\end{align*}

• Thank you. Can you please outline the formula behind doing this type of problem with the limit involved? I haven't learned anything about it in class or in my textbooks. edit: My homework program also says that your answer is incorrect. – user70019 Nov 26 '14 at 21:44
• You are welcome. I edited my answer, is it better now? – Jimmy R. Nov 26 '14 at 21:47
• Yes, this is great. I didn't notice that you changed 981 to 985. – user70019 Nov 26 '14 at 21:51
• Wait, the 985 was a mistake! The 981 is the correct! – Jimmy R. Nov 26 '14 at 21:52
• Hey, this is a great explanation. I was just able to do another one of these by hand. – user70019 Nov 26 '14 at 22:07

Hint1: $$\begin{split} \lim_{n \to \infty }\frac{981}{n+5}\sum_{i=1}^{n} \left(\frac{i^2}{n^2} \right) &= \lim_{n \to \infty} \frac{981}{n+5} \cdot \frac{\sum_{i=1}^{n}i^2}{n^2}\\ \end{split}$$

Hint2:

$$\sum_{i=1}^{n}i^2 = \frac{n\cdot (n+1) \cdot 2n+1)}{6}$$

$$\begin{split} \lim_{n \to \infty }\frac{981}{n+5}\sum_{i=1}^{n} \left(\frac{i^2}{n^2} \right) &= \lim_{n \to \infty} \frac{981}{n+5} \frac{\sum_{i=1}^{n}i^2}{n^2}\\ &= \lim_{n \to \infty}\frac{981}{n+5} \frac{n(n+1)(2n+1)}{6 \cdot n^2}\\ &= \lim_{n \to \infty} \frac{n}{n} \cdot \frac{981 (2n^2+3n+1)}{(6n^2 +30n)}\\ &= \lim_{n \to \infty}\frac{n^2}{n^2} \cdot \frac{981\cdot(2+\frac{3}{n} + \frac{1}{n^2})}{6(1 + \frac{30}{n})} \\ &= \lim_{n \to \infty}\frac{327\cdot(2+\frac{3}{n} + \frac{1}{n^2})}{2(1 + \frac{30}{n})} &= 327 \end{split}$$

• What do I do with the limit? The limit is 0, right? – user70019 Nov 26 '14 at 21:46
• No, 327 as I wrote. Limit of denominator is $2$ and nominator $327 \cdot 2$. Look, $\lim_{n\to\infty}\frac{3}{n} = 0$, $\lim_{n\to\infty}\frac{1}{n^2} = 0$, so $\lim_{n\to\infty}\left(2 + \frac{3}{n} + \frac{1}{n^2}\right) = 2$. And so on. – Tacet Nov 26 '14 at 21:51
• Where does the following come from: $$\sum\limits_{i=1}^n i^2=\frac{n\left(n+1\right)\left(2n+1\right)}{6}$$ – jm324354 Nov 26 '14 at 22:01
• That's the summation formula for i^2. – user70019 Nov 26 '14 at 22:10
• @bd1251252: It's well known fact. Exists so many derivations of this formula. You can find some in Internet, I believe, here too. An interesting way is to note the obvious fact $$(a_2 - a_1) + (a_3 - a_2) + \dots + (a_{n+1} - a_{n}) = a_{n+1}-a_1$$ And let $a_i = \frac{(2i - 1)^3}{24}$. I left you rest. :) – Tacet Nov 26 '14 at 22:18

Hint: Pull the $\frac{1}{n^2}$ out of the sum and use the identity $$\sum_{i=1}^ni^2= \frac{n(n+1)(2n+1)}{6}$$

I see a (calculus) tag. So ... that homework problem must follow a chapter on Riemann sums ... Let's try that.

Consider the integral $$\int_0^1 x^2\,dx = \frac{1}{3} .$$ That integrand $x^2$ is continuous, therefore Riemann integrable. Consider the "upper sums" for $n$ equal-size intervals.

$$\sum_{i=1}^n \left(\frac{i}{n}\right)^2\cdot\frac{1}{n} \to \frac{1}{3}$$ as $n \to \infty$. So the original probem must have answer $981/3 = 327$.
