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Expanding the summation (say S) and comparing it to the familiar Gregory series for arctan ( x ) , we get x=1
OR S = arctan ( 1 ) = 4 π .
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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From expansion series of t a n − 1 x : tan − 1 x = 1 − 3 x 3 + 5 x 5 − 7 x 7 . . . . Put x=1 to get the required summation... ∴ 4 π = n = 1 ∑ ∞ 4 n − 3 1 − 4 n − 1 1 A = 4 ∴ A 2 0 1 6 = 5 0 4 See this for further analysis of this series.