Sum of series or What?

Algebra Level 3

Find the value of :

4 5 + 8 65 + 12 325 + 16 1025 + \large \frac{4}{5} + \frac{8}{65} + \frac{12}{325} + \frac{16}{1025} +\ldots


The answer is 1.

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3 solutions

Romeo Gomez
Apr 3, 2015

We can write the sum as follows 4 4 + 1 + 8 64 + 1 + 12 324 + 1 + 14 1024 + 1 = n = 1 4 n 4 n 4 + 1 , \frac{4}{4+1}+\frac{8}{64+1}+\frac{12}{324+1}+\frac{14}{1024+1}\dots=\sum_{n=1}^{\infty}{\frac{4n}{4n^4+1}}, using the Sophie-Germain equation x 4 + 4 y 4 = ( ( x + y ) 2 + y 2 ) ( ( x y ) 2 + y 2 ) , x = 1 , y = n x^4+4y^4=((x+y)^2+y^2)((x-y)^2+y^2), x=1, y=n we obtain a telescopic serie = n = 1 4 n ( ( 1 n ) 2 + n 2 ) ( ( 1 + n ) 2 + n 2 ) =\sum_{n=1}^{\infty}{\frac{4n}{((1-n)^2+n^2)((1+n)^2+n^2)}} = n = 1 1 ( ( 1 n ) 2 + n 2 ) 1 ( ( 1 + n ) 2 + n 2 ) =\sum_{n=1}^{\infty}{\frac{1}{((1-n)^2+n^2)}-\frac{1}{((1+n)^2+n^2)}} = 1 lim n 1 ( 1 + n ) 2 + n 2 = 1 =1 -\lim_{n\rightarrow \infty}{\frac{1}{(1+n)^2+n^2}}=\boxed{1}

Great solution!

John Frank - 5 years, 3 months ago

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thank you :D

Romeo Gomez - 5 years, 3 months ago
Tanya Gupta
Feb 22, 2014

Split 4 as 5-1....8 as 13-5....12 as 25-13...(1,5)....(5,13)....(13,25)...are factors of the denominator....so we end up forming a telescoping series upto infinity with the first term of 1....which is also the answer!!! Cheers!!

. .
Mar 6, 2021

4 5 + 8 65 + 12 325 + 16 1025 + = 1 \displaystyle \frac 45 + \frac 8 { 65 } + \frac { 12 } { 325 } + \frac { 16 } { 1025 } + \cdots = \boxed { 1 }

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