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Calculus Level 5

0 ( ln x e x ) 2 d x = π A B + γ C D + E γ ln F + G H ( ln I ) J \large \int_0^\infty \left( \dfrac{ \ln x}{e^x } \right)^2 \, dx = \dfrac{\pi^A}B + \dfrac{\gamma^C}D + E \gamma \ln F + \dfrac GH (\ln I)^J

If the equation above holds true, where A , B , C , D , E , F , G , H A,B,C,D,E,F,G,H and I I are positive integers , find A + B + C + D + E + F + G + H + I + J A+B+C+D+E+F+G+H+I+J .

Notation : γ \gamma denotes the Euler-Mascheroni constant , γ 0.5772 \gamma \approx 0.5772 .


The answer is 28.

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2 solutions

Consider the integral J ( a ) = 0 e 2 x x a d x \displaystyle J(a) = \int_{0}^{\infty} e^{-2x}x^{a}dx , which implies J ( a ) = Γ ( a + 1 ) 2 a + 1 \displaystyle J(a) = \frac{\Gamma(a+1)}{2^{a+1}}

We want to evaluate J ( 0 ) \displaystyle J''(0) , J ( a ) = 1 2 a + 1 [ Γ ( a + 1 ) 2 Γ ( a + 1 ) l n 2 + Γ ( a + 1 ) ( l n 2 ) 2 ] \displaystyle J''(a) = \frac{1}{2^{a+1}}[\Gamma''(a+1)-2\Gamma'(a+1)ln2+\Gamma(a+1)(ln2)^2]

Using Γ ( 1 ) = γ , Γ ( 1 ) = γ 2 + π 2 6 \displaystyle \Gamma'(1)=-\gamma,\Gamma''(1)=\gamma^2+\frac{\pi^2}{6} we obtain ,

J ( 0 ) = π 2 12 + γ 2 2 + γ l n 2 + ( l n 2 ) 2 2 \displaystyle J''(0) = \frac{\pi^2}{12}+\frac{\gamma^2}{2}+\gamma ln2 + \frac{(ln2)^2}{2} , Thus A + B + C + D + E + F + G + H + I = 28 \boxed{A+B+C+D+E+F+G+H+I=28}

Mark Hennings
May 22, 2016

The integral is equal to 1 2 0 ( ln u ln 2 ) 2 e u d u = 1 2 [ Γ [ 1 ] 2 ln 2 Γ [ 1 ] + ( ln 2 ) 2 ] \tfrac12\int_0^\infty (\ln u - \ln 2)^2e^{-u}\,du \; = \; \tfrac12\big[\Gamma''[1] - 2\ln 2 \Gamma'[1] + (\ln 2)^2\big] which is equal to 1 12 π 2 + 1 2 γ 2 + γ ln 2 + 1 2 ( ln 2 ) 2 \tfrac1{12}\pi^2 + \tfrac12\gamma^2 + \gamma \ln 2 + \tfrac12(\ln2)^2 This makes the answer 28 \boxed{28} .

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Brilliant Mathematics Staff - 5 years ago

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