As I said in previous post the formal definition of the Riemann Integral is very useful when solving Olympiad problems.

**Problem 1.** Find the following limit
\[\lim_{n\to\infty}\left(\frac{1}{n+1}+\frac{1}{n+2}+...+\frac{1}{2n}\right)\]

*Solution.* Let try to make our sum into something more or less similar to Riemann sum.
\[\sum_{i=1}^n\frac{1}{n+i}=\frac{1}{n}\left(\sum_{i=1}^n\frac{1}{1+\frac{i}{n}}\right).\]

Does this remind you of the monstrous \(\displaystyle\sum_{i=1}^n f(\xi_i)(x_i-x_{i-1})\)? But what if set \(x_i=\dfrac{i}{n}\) and consider the **right Riemann sum**? Now it will transform into \(\dfrac{1}{n}\left(\displaystyle\sum_{i=1}^n f\left(\dfrac{i}{n}\right)\right)\).

From the last formula we can easily understand what function \(f\) we need to consider, so by the definition: \[\lim_{n\to\infty}\left(\frac{1}{n+1}+\frac{1}{n+2}+...+\frac{1}{2n}\right)=\frac{1}{n}\left(\sum_{i=1}^n\frac{1}{1+\frac{i}{n}}\right)=\] \[=\dfrac{1}{n}\left(\displaystyle\sum_{i=1}^n f\left(\dfrac{i}{n}\right)\right)=\int^1_0\frac{1}{x+1}=\boxed{\ln 2}.\]

Now using the same approach try solving the following problems.

**Problem 2.** \[\lim_{n\to\infty}\left(\frac{1}{n+1}+\frac{1}{n+2}+...+\frac{1}{4n}\right)\]

**Problem 3.** \[\lim_{n\to\infty}\left(\frac{n}{n^2+1}+\frac{n}{n^2+4}+...+\frac{n}{n^2+n^2}\right)\]

## Comments

Sort by:

TopNewestProblem 2 is same as Problem 1, the only difference is that the integration limits change. The lower limit is 0 and upper limit is 3. Hence, the answer is \(\ln 4\).

Problem 3 can be written as:

\(\displaystyle \lim_{n\rightarrow \infty} \frac{1}{n}\left(\sum_{r=1}^n \cfrac{1}{1+\left(\frac{r}{n}\right)^2} \right) \)

The above is equivalent to

\(\displaystyle \int_{0}^1 \frac{dx}{1+x^2}=\frac{\pi}{4} \) – Pranav Arora · 2 years, 10 months ago

Log in to reply

– Jncy Rana · 2 years, 10 months ago

bt i dnt undrstand how it is 1 + x^2 & in the problem 1 only x??Log in to reply

– Manish Mishra · 2 years, 10 months ago

Because to get the answer we have to consider a variable (say x) Which will be in form of r/n after modifying the problem.Log in to reply

– Nicolae Sapoval · 2 years, 10 months ago

Good job!Log in to reply

– Weijie Chen · 2 years, 10 months ago

Is this from IMC ?Log in to reply

– Pranav Arora · 2 years, 10 months ago

Sorry but I didn't get your question. Can you please explain what is "IMC"?Log in to reply

– Weijie Chen · 2 years, 10 months ago

It's International Mathematical Competition and it's the IMO of university studensLog in to reply

in problem 3. how we can put the limits from 0 to 3?????????? – Attaullah Khan · 2 years, 9 months ago

Log in to reply