An Easy One

Algebra Level 3

2 x = 1 3 6 x 2 + 6 x = ? 2\sum_{x=1}^{\infty} \frac{3}{6x^{2}+6x} = ?


The answer is 1.

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1 solution

Drex Beckman
Mar 11, 2016

x = 1 3 6 x 2 + 6 x = 1 2 x = 1 1 x 2 + x = 1 2 x = 1 1 x ( x + 1 ) \sum_{x=1}^{\infty} \frac{3}{6x^{2}+6x} = \frac{1}{2} \sum_{x=1}^{\infty} \frac{1}{x^{2}+x} = \frac{1}{2} \sum_{x=1}^{\infty} \frac{1}{x(x+1)} Using partial fraction decomposition : = 1 2 x = 1 1 x 1 ( x + 1 ) =\frac{1}{2} \sum_{x=1}^{\infty} \frac{1}{x}- \frac{1}{(x+1)} The series x = 1 1 x 1 ( x + 1 ) \sum_{x=1}^{\infty} \frac{1}{x}- \frac{1}{(x+1)} converges (telescopes) to 1 1 . Multiplied by 1 2 \frac{1}{2} gives us 1 2 \frac{1}{2} Rounding to the nearest ones gives us 1 \boxed{1} .

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