Partial to Last Year's Integrals

Calculus Level 3

e d x x 2017 x \large \int_e^{\infty} \dfrac{dx}{x^{2017} - x}

The value of the definite integral above is equal to a 1 b ln ( e c d ) , a - \dfrac{1}{b} \ln \left (e^c - d \right ), where a a , b b , c c , and d d are positive integers. Find the value of a + b + c + d . a + b + c + d.

Bonus: Find the general form of the indefinite integral d x x n x \displaystyle \int \dfrac{dx}{x^{n} - x} for positive integers n > 1 n > 1 .


The answer is 4034.

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1 solution

Chew-Seong Cheong
Sep 10, 2018

I = d x x n x = d x x ( x n 1 1 ) = ( x n 2 x n 1 1 1 x ) d x = ln ( x n 1 1 ) n 1 ln x + C where C is the constant of integration. \begin{aligned} I & = \int \frac {dx}{x^n-x} \\ & = \int \frac {dx}{x(x^{n-1}-1)} \\ & = \int \left(\frac {x^{n-2}}{x^{n-1}-1} - \frac 1x \right) dx \\ & = \frac {\ln(x^{n-1}-1)}{n-1} - \ln x + \color{#3D99F6} C & \small \color{#3D99F6} \text{where }C \text{ is the constant of integration.} \end{aligned}

Therefore,

e d x x 2017 x = ln ( x 2016 1 ) 2016 ln x e = lim x ln ( x 2016 1 x 2016 ) 1 2016 ln ( e 2016 1 ) 2016 + ln e = lim x ln ( 1 1 x 2016 ) 1 2016 ln ( e 2016 1 ) 2016 + 1 = 0 + 1 ln ( e 2016 1 ) 2016 \begin{aligned} \int_e^\infty \frac {dx}{x^{2017}-x} & = \frac {\ln(x^{2016}-1)}{2016} - \ln x\ \bigg|_e^\infty \\ & = \lim_{x \to \infty} \ln \left(\frac {x^{2016}-1}{x^{2016}}\right)^{\frac 1{2016}} - \frac {\ln(e^{2016}-1)}{2016} + \ln e \\ & = \lim_{x \to \infty} \ln \left(1 - \frac 1{x^{2016}}\right)^{\frac 1{2016}} - \frac {\ln(e^{2016}-1)}{2016} + 1 \\ & = 0 + 1 - \frac {\ln(e^{2016}-1)}{2016} \end{aligned}

a + b + c + d = 1 + 2016 + 2016 + 1 = 4034 \implies a+b+c+d = 1+2016+2016+1 = \boxed{4034} .

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