Limit of sum of wha?!

lim n 9 n ( 1 8 + 2 8 + 3 8 + 4 8 + 5 8 + . . . + n 8 ) 1 9 + 2 9 + 3 9 + 4 9 + 5 9 + . . . + n 9 = ? \large \lim_{n\to\infty}\dfrac{9n(1^8+2^8+3^8+4^8+5^8+...+n^8)}{1^9+2^9+3^9+4^9+5^9+ ...+ n^9}=?


The answer is 10.

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2 solutions

Chew-Seong Cheong
Apr 17, 2018

Relevant wiki: Sum of n, n², or n³

L = lim n 9 n ( 1 8 + 2 8 + 3 8 + + n 8 ) 1 9 + 2 9 + 3 9 + + n 9 = lim n 9 n k = 1 n k 8 k = 1 n k 9 Using Faulhaber’s formula = lim n 9 n 1 90 ( 10 n 9 + 45 n 8 + 60 n 7 42 n 5 + 20 n 3 3 n ) 1 20 ( 2 n 10 + 10 n 9 + 15 n 8 14 n 6 + 10 n 4 3 n 2 ) = lim n 2 ( 10 n 10 + 45 n 9 + 60 n 8 42 n 6 + 20 n 4 3 n 2 ) 2 n 10 + 10 n 9 + 15 n 8 14 n 6 + 10 n 4 3 n 2 Divide up and down by n 10 = lim n 2 ( 10 + 45 n + 60 n 2 42 n 4 + 20 n 6 3 n 8 ) 2 + 10 n + 15 n 2 14 n 4 + 10 n 6 3 n 8 = 10 \begin{aligned} L & = \lim_{n \to \infty} \frac {9n(1^8+2^8+3^8+\cdots+n^8)}{1^9+2^9+3^9+\cdots+n^9} \\ & = \lim_{n \to \infty} \frac {9n \color{#3D99F6} \sum_{k=1}^n k^8}{\color{#3D99F6}\sum_{k=1}^n k^9} & \small \color{#3D99F6} \text{Using Faulhaber's formula} \\ & = \lim_{n \to \infty} \frac {9n\cdot \color{#3D99F6}\frac 1{90}(10n^9+45n^8+60n^7-42n^5+20n^3-3n)}{\color{#3D99F6}\frac 1{20}(2n^{10}+10 n^9+15n^8-14n^6+10n^4-3n^2)} \\ & = \lim_{n \to \infty} \frac {2(10n^{10}+45n^9+60n^8-42n^6+20n^4-3n^2)}{2n^{10}+10 n^9+15n^8-14n^6+10n^4-3n^2} & \small \color{#3D99F6} \text{Divide up and down by }n^{10} \\ & = \lim_{n \to \infty} \frac {2\left(10+\frac {45}n+ \frac {60}{n^2}- \frac {42}{n^4}+\frac {20}{n^6}-\frac 3{n^8}\right)}{2+\frac {10}n+\frac {15}{n^2}-\frac {14}{n^4}+\frac {10}{n^6}-\frac 3{n^8}} \\ & = \boxed{10} \end{aligned}


Reference: Faulhaber's formula

Timothy Cao
Apr 16, 2018

Not a solution, but it could be a hint towards one of the methods to solve this: https://en.wikipedia.org/wiki/Euler%E2%80%93Maclaurin_formula

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