Tasty summation!

Calculus Level 3

Evaluate : n = 1 2 n 3 n + 1 . \sum_{n=1}^{\infty} \dfrac {2n}{3^{n+1}}.


The answer is 0.5.

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1 solution

Mateus Gomes
Dec 29, 2015

n = 1 2 n 3 n + 1 \sum_{n=1}^{\infty} \dfrac {2n}{3^{n+1}} 2 3 n = 1 n 3 n \frac{2}{3}\sum_{n=1}^{\infty} \dfrac {n}{3^{n}} n = 1 n 3 n S 1 = 1 3 + 2 3 2 + 3 3 3 + 4 3 4 . . . ( 1 ) \sum_{n=1}^{\infty} \dfrac {n}{3^{n}}\rightarrow S_1=\frac{1}{3}+\frac{2}{3^2}+\frac{3}{3^3}+\frac{4}{3^4}...~~(1) S 1 3 = 1 3 2 + 2 3 3 + 3 3 4 + 4 3 5 . . . ( 2 ) ~~~~~~~~~~~\rightarrow \frac{S_1}{3}=\frac{1}{3^2}+\frac{2}{3^3}+\frac{3}{3^4}+\frac{4}{3^5}...~~(2) 2 S 1 3 = 1 3 + 1 3 2 + 1 3 3 + 1 3 4 + 1 3 5 . . . ( 1 ) ( 2 ) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\rightarrow \frac{2S_1}{3}=\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+\frac{1}{3^4}+\frac{1}{3^5}...~~(1)-(2) 2 S 1 3 = 1 3 1 1 3 = 1 2 \rightarrow \frac{2S_1}{3}=\frac{\frac{1}{3}}{1-\frac{1}{3}}=\frac{1}{2} S 1 = 3 4 \rightarrow S_1=\frac{3}{4} 2 3 n = 1 n 3 n = 2 3 S 1 = 2 3 × 3 4 = \frac{2}{3}\sum_{n=1}^{\infty} \dfrac {n}{3^{n}}=\frac{2}{3}S_1=\frac{2}{3}\times\frac{3}{4}= n = 1 2 n 3 n + 1 = 1 2 \sum_{n=1}^{\infty} \dfrac {2n}{3^{n+1}}=\Large\color{#3D99F6}{\boxed{\color{forestgreen}{\boxed{\frac{1}{2}}}}}

Oh very nice . did same..

Rakshit Joshi - 4 years, 8 months ago

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