Is it a telescopic series?

Calculus Level 4

If π A = n = 1 ( 1 4 n 3 1 4 n 1 ) \displaystyle \dfrac \pi A = \sum_{n=1}^\infty \left( \dfrac1{4n-3} - \dfrac1{4n-1} \right) , compute 2016 A \dfrac{2016}A .


Inspiration .


The answer is 504.

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3 solutions

Rishabh Jain
Feb 11, 2016

From expansion series of t a n 1 x tan^{-1} x : tan 1 x = 1 x 3 3 + x 5 5 x 7 7 . . . . \tan^{-1} x=1-\dfrac{x^3}{3} +\dfrac{x^5}{5}- \dfrac{x^7}{7}.... Put x=1 to get the required summation... π 4 = n = 1 1 4 n 3 1 4 n 1 \Large\therefore~\dfrac{\pi}{4}=\sum _{ n=1 }^{ \infty }\frac { 1 }{ 4n-3 } -\frac { 1 }{ 4n-1 } A = 4 \Large A=4 2016 A = 504 \huge\therefore \dfrac{2016}{A}=\boxed{\color{#007fff}{504}} See this for further analysis of this series.

Pulkit Gupta
Feb 11, 2016

Expanding the summation (say S) and comparing it to the familiar Gregory series for arctan ( x ) \arctan(x) , we get x=1

OR S = arctan ( 1 ) \arctan(1) = π 4 \frac{\pi}{4} .

Prakhar Bindal
May 30, 2016

Firstly We can write 1/4n-3 and 1/4n-1 as integral from 0 to 1 of x^(4n-4) and x^(4n-2).

Then interchanging summation and integration we will get two geometric progressions which on simplification will yield

Integration from 0 to 1 of 1/(1+x^2) which gives required result

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