# I don't understand the highlighted step in this integral exercise

#### egal

##### New member

Where did that 2n^3+3n^2+n over 6 come from?

#### canvas

##### New member
It should looks like that, there shouldn't be 2/n^2 but 1/n^2, do you get it now?

$\frac{1}{n^3}\cdot\frac{2n^3+3n^2+n}{6}=\frac{2n^3+3n^2+n}{6n^3}=\frac{\frac{2n^3}{n^3}+\frac{3n^2}{n^3}+\frac{n}{n^3}}{6}=\frac{2+\frac{3}{n}+\frac{1}{n^2}}{6}$

#### lev888

##### Senior Member
The sum of consecutive squares formula. Then we divided the 3 terms in the numerator by n^3. There is a typo: the last 2 should be 1.

#### egal

##### New member
It should looks like that, there shouldn't be 2/n^2 but 1/n^2, do you get it now?

$\frac{1}{n^3}\cdot\frac{2n^3+3n^2+n}{6}=\frac{2n^3+3n^2+n}{6n^3}=\frac{\frac{2n^3}{n^3}+\frac{3n^2}{n^3}+\frac{n}{n^3}}{6}=\frac{2+\frac{3}{n}+\frac{1}{n^2}}{6}$
The sum of consecutive squares formula. Then we divided the 3 terms in the numerator by n^3. There is a typo: the last 2 should be 1.
okayyy, I read about sum of consecutive squares formula and got it, Thanks a lot!

#### canvas

##### New member
okayyy, I read about sum of consecutive squares formula and got it, Thanks a lot!
There you are! These formulas are pretty useful:

$\sum_{i=1}^n{i}=\frac{n(n+1)}{2}\\\\\sum_{i=1}^n{i^2}=\frac{n(n+1)(2n+1)}{6}\\\\\sum_{i=1}^n{i^3}=\frac{n^2(n+1)^2}{4}\\\\\text{P.S. You can prove them by induction ;)}$
Have a nice day!

#### egal

##### New member
There you are! These formulas are pretty useful:

$\sum_{i=1}^n{i}=\frac{n(n+1)}{2}\\\\\sum_{i=1}^n{i^2}=\frac{n(n+1)(2n+1)}{6}\\\\\sum_{i=1}^n{i^3}=\frac{n^2(n+1)^2}{4}\\\\\text{P.S. You can prove them by induction ;)}$
Have a nice day!
thanks a lot again, you too!