Who's up to the challenge? 50

Calculus Level 5

0 ln x 50 e x 2 + 1 d x = π ( a γ + ln b ) c e \large \int _{ 0 }^{ \infty }{ \frac { \ln { \sqrt [ 50 ]{ x } } }{ { e }^{ { x }^{ 2 }+1 } } dx } =-\dfrac { \sqrt { \pi } \left( a\gamma +\ln { b } \right) }{ ce }

If the above equation holds true for positive integers a , b a,b and c c , find a + b + c a+b+c .

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


The answer is 205.

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1 solution

Consider the integral : J ( a ) = 0 e ( x 2 + 1 ) x a d x \displaystyle J(a)=\int_{0}^{\infty} e^{-(x^2+1)}x^a dx , where we want to evaluate J ( 0 ) J'(0)

J ( a ) = Γ ( a + 1 2 ) 2 e \displaystyle J(a) = \frac{\Gamma(\frac{a+1}{2})}{2e} , Differentiating with respect to a a we get ,

J ( a ) = 1 4 e [ Γ ( a + 1 2 ) ψ ( 0 ) ( a + 1 2 ) ] \displaystyle J'(a) = \frac{1}{4e} [\Gamma(\frac{a+1}{2})\psi_{(0)}(\frac{a+1}{2})] , which implies J ( 0 ) = π ( γ + l n 4 ) 4 e \displaystyle J'(0)=-\frac{\sqrt{\pi}(\gamma+ln4)}{4e}

We have , 0 ln x 50 e x 2 + 1 d x = 1 50 J ( 0 ) = π ( γ + l n 4 ) 200 e \displaystyle \int _{ 0 }^{ \infty }{ \frac { \ln { \sqrt [ 50 ]{ x } } }{ { e }^{ { x }^{ 2 }+1 } } dx } = \frac{1}{50}J'(0)=-\frac{\sqrt{\pi}(\gamma+ln4)}{200e}

Thus making the answer : 205 \boxed{205}

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