A Fancy Limit

Calculus Level 2

lim n k = 1 n k 5 n 8 = ? \large \displaystyle \lim_{n \to \infty} \dfrac{ \displaystyle \sum_{k=1}^{n} k^5}{n^8}= \, ?

1 3 \dfrac{1}{3} 1 1 0 0 None of these choices 1 6 \dfrac{1}{6}

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Rishabh Jain
Apr 6, 2016

Limit can be written as: ( lim n 1 n 2 ) × ( lim n 1 n k = 1 n k 5 n 5 ) \left(\displaystyle\lim_{n\to \infty}\dfrac{1}{n^2}\right)\times \left(\displaystyle\lim_{n\to \infty}\dfrac{1}{n}\sum_{k=1}^{n}\dfrac{k^5}{n^5}\right)

Using Reimann Sums the second limit can be calculated (as 1 6 \frac 16 ) while first limit is obviously 0 0 . Hence overall limit is 0 0 .

L = lim n k = 1 n k 5 n 8 = lim n 1 12 ( 2 n 6 + 6 n 5 + 5 n 4 n 2 ) n 8 See Note = lim n ( 1 6 n 2 + 1 2 n 3 + 5 15 n 4 1 12 n 6 ) = 0 \begin{aligned} \mathfrak {L} & = \lim_{n \to \infty} \frac{\small {\displaystyle \sum_{k=1}^n k^5}}{n^8} \\ & = \lim_{n \to \infty} \frac{\color{#3D99F6}{\frac{1}{12}(2n^6+6n^5+5n^4-n^2)}}{n^8} \quad \quad \quad \quad \small \color{#3D99F6}{\text{See Note}} \\ & = \lim_{n \to \infty} \left( \frac{1}{6n^2} + \frac{1}{2n^3} + \frac{5}{15n^4} - \frac{1}{12n^6} \right) \\ & = \boxed{0} \end{aligned}

Note: \color{#3D99F6}{\text{Note:}} See equation (25) of Power Sum .

Can you please explain the expansion used in the second step?

monty g - 5 years, 2 months ago

Log in to reply

See equation (25) of Power Sum .

Chew-Seong Cheong - 5 years, 2 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...