Integrate all powers of log

Calculus Level 5

Consider F ( a ) = 0 1 ln a ( x ) d x . F(a)=\int_0^1 \ln^a(x) \, dx . Find ln ( a = 0 1 F ( a ) ) . \ln\left(\sum_{a=0}^\infty\dfrac{1}{F(a)}\right).


The answer is -1.

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2 solutions

Rishabh Jain
Feb 19, 2016

By Integration by Parts: F ( a ) = x ln a x 0 1 a 0 1 ln a 1 ( x ) d x \large F(a)=x \ln^a x|_{\small 0}^{\small 1} - a\int_0^1 \ln^{a-1} (x) \, dx = a F ( a 1 ) ( x ln a x 0 1 = 0 ) \large =-aF(a-1)~~\small{(\color{#3D99F6}{\because x \ln^a x|_{\small 0}^{\small 1}=0})} = a ( a 1 ) F ( a 2 ) \large =a(a-1)F(a-2) = a ( a 1 ) ( a 2 ) F ( a 3 ) \large =-a(a-1)(a-2)F(a-3) \large \cdots\cdots = ( 1 ) a a ! F ( 0 ) = ( 1 ) a a ! ( F ( 0 ) = 1 ) \large =(-1)^a a! F(0)=(-1)^a a! ~~\small{(\color{#3D99F6}{\because F(0)=1})} ln ( a = 0 1 F ( a ) ) = ln ( 1 e ) = 1 \large \therefore \ln\left(\sum_{a=0}^\infty\dfrac{1}{F(a)}\right)=\ln(\dfrac{1}{e})=\huge \boxed{\color{#007fff}{-1}}


N O T E : Consider expansion of e x : e x = 1 + x 1 ! + x 2 2 ! + x 3 3 ! \mathcal{NOTE:}\\ \text{Consider expansion of }e^x :\\ \color{#D61F06}{e^x=1+\frac{x}{1!}+\frac{x^2}{2!}+\frac{x^3}{3!}\cdots} Putting x=-1, we get 1 e = a = 0 1 ( 1 ) a a ! = a = 0 1 F ( a ) \dfrac{1}{e}= \sum_{a=0}^\infty\dfrac{1}{(-1)^a a!}=\sum_{a=0}^\infty\dfrac{1}{F(a)}

Aareyan Manzoor
Feb 19, 2016

consider 0 1 x n dx = 1 n + 1 \int_0^1 x^n \text{dx} = \dfrac{1}{n+1} d.w.r.n a time and putting n = 0 to get 0 1 ln a ( x ) dx = ( 1 ) a a ! \int_0^1 \ln^a(x) \text{dx}=(-1)^a a! now put this in the summation a = 0 ( 1 ) a a ! = e 1 \sum_{a=0}^\infty \dfrac{(-1)^a}{a!}=e^{\boxed{-1}} In the last step the taylor series of e x e^x was used.

Moderator note:

Can you explain how we "d.w.r.n a time and putting n = 0"?

Can you explain how we "d.w.r.n a time and putting n = 0"?

Calvin Lin Staff - 5 years, 3 months ago

I was gonna write a solution, but you already wrote the exact method I used

A Former Brilliant Member - 5 years, 3 months ago

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