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Calculus Level 5

2 1 ln ( x 2 x ) x d x = 1 A ( π B C ln D F ) + L i 2 ( G ) \displaystyle\int _{ -2 }^{ -1 }{ \dfrac { \ln { { (x }^{ 2 }-x) } }{ x } } \, dx =\frac { 1 }{ A } ({ \pi }^{ B }-C{ \ln { ^{ D }{ F } } })+{ Li }_{ 2 }(-G)

where A , B , C , D , F , G A,B,C,D,F,G are positive integers with F F minimized

find A + B + C + D + F + G A+B+C+D+F+G

Notation : Li n ( a ) { \text{Li} }_{ n }(a) denotes the polylogarithm function, Li n ( a ) = k = 1 a k k n . { \text{Li} }_{ n }(a)=\displaystyle\sum _{ k=1 }^{ \infty }{ \frac { { a }^{ k } }{ { k }^{ n } } }.


The answer is 26.

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1 solution

I = 2 1 ln ( x 2 x ) x d x Let u = x = 2 1 ln ( u 2 + u ) u d u = 2 1 ln u + ln ( 1 + u ) u d u = 2 1 ln u u d u + 2 1 ln ( 1 + u ) u d u By integration by parts = [ ln 2 u ] 2 1 2 1 ln u u d u + 2 1 1 u k = 1 ( 1 ) k + 1 u k k d u Maclaurin series = ln 2 2 2 + 2 1 k = 1 ( 1 ) k + 1 u k 1 k d u = ln 2 2 2 + k = 1 ( 1 ) k + 1 2 1 u k 1 k d u = ln 2 2 2 + k = 1 ( 1 ) k + 1 [ u k k 2 ] 2 1 = ln 2 2 2 + k = 1 ( 1 ) k + 1 1 k 2 k = 1 ( 1 ) k + 1 2 k k 2 = ln 2 2 2 + 1 2 k = 1 1 k 2 + k = 1 ( 2 ) k k 2 = ln 2 2 2 + π 2 12 + L i 2 ( 2 ) = 1 12 ( π 2 6 ln 2 2 ) + L i 2 ( 2 ) \begin{aligned} I & = \int_{-2}^{-1} \frac{\ln (x^2-x)}{x} dx \quad \quad \small \color{#3D99F6}{\text{Let }u = - x} \\ & = \int_{2}^{1} \frac{\ln (u^2+u)}{u} du \\ & = \int_{2}^{1} \frac{\ln u + \ln (1+u)}{u} du \\ & = \color{#3D99F6}{\int_{2}^{1} \frac{\ln u}{u} du} + \int_{2}^{1} \frac{\color{#D61F06}{\ln (1+u)}}{u} du \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \small \color{#3D99F6}{\text{By integration by parts}} \\ & = \color{#3D99F6}{\left[\ln^2 u \right]_2^1 - \int_{2}^{1} \frac{\ln u}{u}du} + \int_{2}^{1} \frac{1}{u} \color{#D61F06}{\sum_{k=1} ^\infty(-1)^{k+1} \frac{u^k}{k}} du \quad \quad \space \small \color{#D61F06}{\text{Maclaurin series}} \\ & = \color{#3D99F6}{-\frac{\ln^2 2}{2}} + \int_{2}^{1} \sum_{k=1} ^\infty(-1)^{k+1} \frac{u^{k-1}}{k} du \\ & = -\frac{\ln^2 2}{2} + \sum_{k=1}^\infty(-1)^{k+1} \int_{2}^{1} \frac{u^{k-1}}{k} du \\ & = -\frac{\ln^2 2}{2} + \sum_{k=1}^\infty(-1)^{k+1} \left[\frac{u^k}{k^2}\right]_2^1 \\ & = -\frac{\ln^2 2}{2} + \sum_{k=1}^\infty(-1)^{k+1} \frac{1}{k^2} - \sum_{k=1}^\infty(-1)^{k+1} \frac{2^k}{k^2} \\ & = -\frac{\ln^2 2}{2} + \frac{1}{2} \sum_{k=1}^\infty \frac{1}{k^2} + \sum_{k=1}^\infty \frac{(-2)^k}{k^2} \\ & = -\frac{\ln^2 2}{2} + \frac{\pi^2}{12} + Li_2 (-2) \\ & = \frac{1}{12}\left(\pi^2-6\ln^2 2\right) + Li_2 (-2) \end{aligned}

A + B + C + D + F + G = 12 + 2 + 6 + 2 + 2 + 2 = 26 \Rightarrow A+B+C+D+F+G = 12+2+6+2+2+2 = \boxed{26}

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