What will be the sum?

Calculus Level 3

1 1 4 + 1 8 1 10 + 1 14 1 16 + = ? 1-\frac{1}{4} +\frac{1}{8} -\frac{1}{10} + \frac{1}{14} -\frac{1}{16} + \cdots = \ ?

The problem is original


The answer is 0.8022.

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2 solutions

Dwaipayan Shikari
Nov 27, 2020

I found this interesting \textrm{I found this interesting}

t a n x = 1 π 2 x 1 π 2 + x + 1 3 π 2 x 1 3 π 2 + x + 1 5 π 2 x \mathrm{tanx = \frac{1}{\frac{π}{2}-x} -\frac{1}{\frac{π}{2}+x} +\frac{1}{\frac{3π}{2}-x}-\frac{1}{\frac{3π}{2}+x} + \frac{1}{\frac{5π}{2}-x } -\cdots}

This lemma is a consequence of "Euler’s product" formula \textrm{ This lemma is a consequence of "Euler's product" formula}

taking \textrm{taking} x = π 6 \mathrm{x= \frac{π}{6}}

We will end up with \textrm{We will end up with}

t a n ( π 6 ) = 6 2 π 6 4 π + 6 8 π 6 10 π + 6 14 π \mathrm{ tan(\frac{π}{6}) = \frac{6}{2π} -\frac{6}{4π} + \frac{6}{8π} - \frac{6}{10π} + \frac{6}{14π} -\cdots}

So \textrm{So} π 6 3 = 1 2 1 4 + 1 8 1 10 + 1 14 1 16 + \mathrm{\frac{π}{6√3} = \frac{1}{2} -\frac{1}{4} +\frac{1}{8} -\frac{1}{10} + \frac{1}{14} -\frac{1}{16} +\cdots }

By Adding \textrm{By Adding} 1 2 \frac{1}{2} we can get our answer \textrm{we can get our answer}

Answer will be π 6 3 + 1 2 \textrm{Answer will be \boxed { \frac{π}{6√3} +\frac{1}{2} } }

There are infinitely many sequences that can be generated from 1, -4, +8, ... How were we to know you meant this one?

Fletcher Mattox - 6 months, 2 weeks ago
Tom Engelsman
Nov 27, 2020

The above series can be written as:

1 + Σ n = 0 1 6 n + 8 1 6 n + 4 ; 1 + \Sigma_{n=0}^{\infty} \frac{1}{6n+8} - \frac{1}{6n+4};

or 1 + Σ n = 0 0 1 x 6 n + 7 x 6 n + 3 d x 1 + \Sigma_{n=0}^{\infty} \int_{0}^{1} x^{6n+7} - x^{6n+3} dx ;

or 1 + 0 1 ( x 7 x 3 ) Σ n = 0 x 6 n d x ; 1 + \int_{0}^{1} (x^7-x^3) \cdot \Sigma_{n=0}^{\infty} x^{6n} dx;

or 1 + 0 1 x 7 x 3 1 x 6 d x 1 + \int_{0}^{1} \frac{x^7-x^3}{1-x^6} dx ;

or 1 0 1 x 3 ( x 4 1 ) ( x 3 + 1 ) ( x 3 1 ) d x ; 1 - \int_{0}^{1} \frac{x^3(x^4-1)}{(x^3+1)(x^3-1)} dx;

or 1 0 1 x 3 ( x 2 + 1 ) ( x + 1 ) ( x 1 ) ( x + 1 ) ( x 2 x + 1 ) ( x 1 ) ( x 2 + x + 1 ) d x 1 - \int_{0}^{1} \frac{x^3(x^2+1)(x+1)(x-1)}{(x+1)(x^2-x+1)(x-1)(x^2+x+1)} dx ;

or 1 0 1 x 5 + x 3 x 4 + x 2 + 1 d x 1 - \int_{0}^{1} \frac{x^5+x^3}{x^4+x^2+1} dx ;

or 1 0 1 x x x 4 + x 2 + 1 d x 1 - \int_{0}^{1} x - \frac{x}{x^4+x^2+1} dx ;

or 1 0 1 x x ( x 2 + 1 / 2 ) 2 + 3 / 4 d x 1 - \int_{0}^{1} x - \frac{x}{(x^2+1/2)^2 + 3/4} dx ;

or 1 [ x 2 2 1 3 arctan ( 2 x 2 + 1 3 ) 0 1 ] ; 1 - [\frac{x^2}{2} - \frac{1}{\sqrt{3}} \arctan (\frac{2x^2+1}{\sqrt{3}})|_{0}^{1}];

or 1 2 + π 6 3 . \boxed{\frac{1}{2} + \frac{\pi}{6\sqrt{3}}}.

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