1. Nov 21, 2012

### Artusartos

I don't really understand why $\frac{b^3}{n^3} \frac{n(n+1)(2n+1)}{6}$ is close to b^3/3 if n is very large...can anybody explain this to me?

I attached the problem...

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2. Nov 21, 2012

### Robert1986

If $n$ is big, then $\frac{n(n+1)(2n+1)}{6} \simeq 2n^3/6$ (in the sense that as $n \to \infty$ we have $\frac{n(n+1)(2n+1)}{6} \to \frac{2n^3}{6}$).

3. Nov 21, 2012

### Staff: Mentor

Expand n(n + 1)(2n + 1), and then factor out n3. Finally, take the limit of your expression as n gets large.

4. Nov 21, 2012

### Ray Vickson

In other words, write
$$\frac{n(n+1)(2n+1)}{n^3} = \frac{2 n^3}{n^3} \left(1 + \frac{1}{n}\right) \left( 1 + \frac{1}{2n} \right).$$

RGV

5. Nov 21, 2012

Thanks :)