My First Problem

Algebra Level 3

y = x = 1 ( 5 x 1 + 2 x 5 x 5 5 + x 4 x 6 ) y = \sum_{x=1}^\infty \left( \dfrac{5x^{-1} + 2x^{-5}}{x^5} - \dfrac{5+x^{-4}}{x^6}\right)

Find the value of y y to the nearest whole number (i.e. y \left \lfloor y \right \rceil ).


The answer is 1.

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1 solution

Drex Beckman
Dec 16, 2015

First, start by finding a common denominator: x 6 x 6 5 x 1 + 2 x 5 x 5 x 5 x 5 5 + x 4 x 6 \frac{x^6}{x^6}\cdot \frac{5x^{-1}+2x^{-5}}{x^{5}}-\frac{x^5}{x^5}\cdot \frac{5+x^{-4}}{x^{6}} This becomes: 5 x 5 + 2 x 1 x 11 5 x 5 + x 1 x 11 \frac{5x^{5}+2x^{1}}{x^{11}}-\frac{5x^{5}+x^{1}}{x^{11}} You then simplify: 5 x 5 + 2 x 1 5 x 5 x 1 x 11 = x x 11 \frac{5x^{5}+2x^{1}-5x^{5}-x^{1}}{x^{11}} = \frac{x}{x^{11}} Get rid of like terms: 1 x 10 \frac{1}{x^{10}} You are left with: y = ζ ( 10 ) = i = 1 1 x 10 1.00099 y = \zeta(10) = \sum_{i = 1}^{\infty} \frac{1}{x^{10}} \approx 1.00099 Rounding to the nearest whole leaves us with 1.

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