An amazing summation! (part-2)

Calculus Level 2

n = 1 8 16 n 2 16 n + 3 = ? \large\sum_{n=1}^{\infty}\frac8{16n^2-16n+3}=\ ?

Give your answer to 2 decimal places.


The answer is 3.14.

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1 solution

Satyajit Mohanty
Jul 14, 2015

n = 1 8 16 n 2 16 n + 3 = n = 1 8 ( 4 n 1 ) ( 4 n 3 ) \displaystyle \sum_{n=1}^{\infty}\dfrac8{16n^2-16n+3} = \displaystyle \sum_{n=1}^{\infty}\dfrac8{(4n-1)(4n-3)}

By turning it into partial fractions, we obtain:

= 4 n = 1 ( 1 4 n 3 1 4 n 1 ) = \displaystyle 4\sum_{n=1}^{\infty}(\dfrac1{4n-3} - \dfrac1{4n-1})

= 4 ( 1 1 1 3 + 1 5 1 7 + 1 9 ad inf ) =\large{4(\dfrac1{1} - \dfrac{1}{3} + \dfrac{1}{5} - \dfrac{1}{7} + \dfrac{1}{9} - \ldots \text { ad inf})}

We know by Gregory's Series that:

tan 1 ( x ) = x x 3 3 + x 5 5 x 7 7 + ad inf \large{\tan^{-1} (x) = x - \frac{x^3}{3} + \frac{x^5}{5} - \frac{x^7}{7} + \ldots \text { ad inf}}

Therefore, putting x = 1 x=1 in the above series, we obtain the next step of solution as

= 4 tan 1 ( 1 ) = 4 ( π 4 ) = π = 3.14 = 4\tan^{-1} (1) = 4(\dfrac{\pi}{4}) = \pi = \boxed{3.14}

Moderator note:

It's better to mention that you perform partial fraction. For clarity, can you explain why π 4 = 1 1 1 3 + 1 5 1 7 + \frac{\pi}4 = \frac11 - \frac13 + \frac15-\frac17 + \ldots ?

@ChallengeMaster, Edited! :)

Satyajit Mohanty - 5 years, 11 months ago

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