# Limit of $\lim_{n\rightarrow \infty} \sum^{n}_{i=1} \frac{1}{\sqrt {n^2 +i} }$

$$\lim_{n\rightarrow \infty} \sum^{n}_{i=1} \frac{1}{\sqrt {n^2 +i} } = \lim_{n\rightarrow \infty} \sum^{n}_{i=1} \frac{1}{n \sqrt {1 +\frac{i}{n^2}} } =$$

$$= \lim_{n\rightarrow \infty} \frac{1}{n} \cdot \lim_{n\rightarrow \infty} \sum^{n}_{i=1} \frac{1}{\sqrt {1 +\frac{i}{n^2}} } = 0 \cdot \sum^{n}_{i=1} 1 = 0 \cdot n$$

So, I tried to apply squeeze theorem:

$$\lim_{n\rightarrow \infty} \sum^{n}_{i=1} \frac{1}{\sqrt {n^2 +i} } \leq \lim_{n\rightarrow \infty} \sum^{n}_{i=1} \frac{1}{n} = 1$$

Could you help me to figure out the lower bound?

• you can't take the product of the limits as you did in step 3
– Alex
Jun 16 '14 at 11:57
• Why? Both limits are defined.
– user
Jun 16 '14 at 11:58
• are you sure the denominator is not $\sqrt{n^2-i^2}$?
– Alex
Jun 16 '14 at 11:58
• Squeezing is very good. What is the smallest term in the sum? Jun 16 '14 at 12:00
• the limit $\frac{1}{\sqrt{1+\frac{i}{n^2}}}=1$, so the sum diverges
– Alex
Jun 16 '14 at 12:03

Notice that $$\frac{n}{\sqrt{n^2+n}} =\sum_{i=1}^n \frac{1}{\sqrt{n^2+n}}\leq \sum_{i=1}^n\frac{1}{\sqrt{n^2+i}}\leq \sum_{i=1}^n \frac{1}{n} =1$$ Since $\frac{n}{\sqrt{n^2+n}}=\frac{1}{\sqrt{1+\frac{1}{n}}}\to 1$, you get that $1$ as your limit by squeeze theorem.