Who's up to the challenge?15

Calculus Level 5

0 ( x x 2 + 1 ) 5 [ ln ( x x 2 + 1 ) ] d x x 2 = A B \displaystyle\int _{ 0 }^{ \infty }{ (\frac { x }{ { x }^{ 2 }+1 } )^{ 5 }[\ln { ( } \frac { x }{ { x }^{ 2 }+1 } )]\frac { dx }{ { x }^{ 2 } } = } -\frac { A }{ B }

If the equation above holds true for coprime positive integers A , B , A,B, , find B A 104 B-A-104 .


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The answer is 35.

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1 solution

Mark Hennings
Feb 22, 2016

The integral is equal to F ( 5 ) F'(5) , where F ( a ) = 0 x a 2 ( x 2 + 1 ) a d x = 1 2 0 u 1 2 ( a 1 ) ( 1 + u ) a d a = 1 2 B ( 1 2 ( a 1 ) , 1 2 ( a + 1 ) ) F(a) \; = \; \int_0^\infty \frac{x^{a-2}}{(x^2+1)^a}\,dx \; =\; \tfrac12\int_0^\infty \frac{u^{\frac12(a-1)}}{(1+u)^a}\,da \; = \; \tfrac12B\big(\tfrac12(a-1),\tfrac12(a+1)\big) and so F ( 5 ) = F ( 5 ) [ 1 2 ψ ( 2 ) + 1 2 ψ ( 3 ) ψ ( 5 ) ] = 1 24 [ 1 2 H 1 + 1 2 H 2 H 4 ] = 1 24 [ H 2 H 4 1 4 ] F'(5) \; =\;F(5)\big[\tfrac12\psi(2) + \tfrac12\psi(3) - \psi(5)\big] \; = \; \tfrac{1}{24}\big[\tfrac12H_1 + \tfrac12H_2 - H_4\big] \; = \; \tfrac{1}{24}\big[H_2 - H_4 - \tfrac14\big] making the answer 2 + 4 + 1 + 4 + 24 = 35 2 + 4 + 1 + 4 + 24 \,=\, \boxed{35} , but why give the answer like this? The quantity F ( 5 ) F'(5) is equal to 5 144 -\tfrac{5}{144} . You could even keep the answer the same by saying that the integral is of the form A B -\tfrac{A}{B} where A , B A,B are coprime integers, and ask for B + 31 A \tfrac{B+31}{A} (or something similar) as the answer.

i've edited the problem :)

Hamza A - 5 years, 3 months ago

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But why ask for an answer which does not use B?

Mark Hennings - 5 years, 3 months ago

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i'll include B

Hamza A - 5 years, 3 months ago

How about now?

Hamza A - 5 years, 3 months ago

I got the solution but couldnt answer it in the given form. You should make the changes earlier. Points lost 😭 Btw, you post nice problems!

Rudraksh Shukla - 5 years, 3 months ago

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happy that you enjoyed!:)

Hamza A - 5 years, 3 months ago

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