Integration -3

Calculus Level 5

If I n = 0 1 ( ln x ) n x d x \displaystyle I_n = \int_0^1 \dfrac{(\ln x)^n }{\sqrt x} \, dx , then of which of the following is/are true?

Choose the most precise option.

(A): I n I_n has a finite value for all positive integers n n .
(B): I 2 I 1 + I 3 I 2 + I 4 I 3 = 4 \dfrac{I_2}{I_1} + \dfrac{I_3}{I_2} + \dfrac{I_4}{I_3} = -4 .
(C): I 4 = 9 I_4 = -9 .
(D): I 8 = 40331 I_8 = 40331 .

A A,B,D A,B A,B,C

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2 solutions

Guilherme Niedu
Apr 24, 2017

I ( n ) = 0 1 ln ( x ) n x d x \large \displaystyle I(n) = \int_0^1 \frac{ \ln(x)^n}{\sqrt{x}} dx

ln ( x ) = u d x = x d u = e u d u \color{#20A900} \large \displaystyle \ln(x) = u \rightarrow dx = x \cdot du = e^u \cdot du

I ( n ) = 0 u n e u 2 d u \large \displaystyle I(n) = \int_{-\infty}^0 u^n \cdot e^{\frac{u}{2}} du

Integrating by parts:

I ( n ) = 2 u n e u 2 0 2 0 n u n 1 e u 2 d u \large \displaystyle I(n) = 2 \cdot u^n \cdot e^{\frac{u}{2}} \Bigg |_{- \infty}^0 - 2 \cdot \int_{-\infty}^0 nu^{n-1} \cdot e^{\frac{u}{2}}du

I ( n ) = 2 0 n u n 1 e u 2 d u \large \displaystyle I(n) = - 2 \cdot \int_{-\infty}^0 nu^{n-1} \cdot e^{\frac{u}{2}}du

Repeat it n 1 n-1 more times, totalizing n n times and you'll get:

I ( n ) = ( 1 ) n n ! 2 n 0 e u 2 d u \large \displaystyle I(n) = (-1)^n \cdot n! \cdot 2^n \int_{-\infty}^0 e^{\frac{u}{2}} du

I ( n ) = ( 1 ) n n ! 2 n + 1 \color{#20A900} \boxed {\large \displaystyle I(n) = (-1)^n \cdot n! \cdot 2^{n+1} }

So, both ( C ) (C) and ( D ) (D) are false. Also:

I ( n ) I ( n 1 ) = 2 n \color{#20A900} \boxed {\large \displaystyle \frac{I(n)}{I(n-1)} = -2n}

So, ( B ) (B) is also false. The only option which is true is ( A ) \color{#3D99F6} \boxed{(A)}

It would have been better if we would have applied Gamma in the second step by taking u 2 = z \dfrac{-u}{2}=z

Md Zuhair - 2 years, 8 months ago
Avi Solanki
Apr 24, 2017

@Indraneel Mukhopadhyaya

the relation i get between I(n) and I(n-1) is that their ratio is -2n . but i am not able to comment on option D and C

I think that's rather incorrect.One can clearly transform the integral to the form of I = 2 n 2 ( l n t ) n d t I=2^n*2*\int (lnt)^ndt .This follows on substituting x = t √x=t .This eliminates C , D C,D as I ( n ) I(n) must be even atleast(even if negative).Now I ( n ) I(n) would be defined that's for sure since for every evaluation of I ( n ) I(n) there would be a term t f ( l n t ) tf(lnt) .And the lower limit vanishes the term.And to mention one could just calc I ( 1 ) = 4 , I ( 2 ) = 16 , I ( 3 ) = 96 I(1)=-4,I(2)=16,I(3)=-96 This implying the ratio of I ( n ) / I ( n 1 ) I(n)/I(n-1) gets bigger and bigger.And thus B is incorrect.What lead you to the deduction I ( n ) / I ( n 1 ) = 2 n I(n)/I(n-1)=2n .That's wrong for sure.

Spandan Senapati - 4 years, 1 month ago

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i wrote (x)^-(1/2) as -2d(x^(1/2)) and applied udv

avi solanki - 4 years, 1 month ago

Here I(n+1)/I(n)= -2(n+1) By applying integration by parts

Devansh Jain - 4 years, 1 month ago

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the same relation i got .put n as n-1 and ull see

avi solanki - 4 years, 1 month ago

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@Avi Solanki I wanted to upload solution for the question (3D and limits).. Can u tell me how to upload images..

Devansh Jain - 4 years, 1 month ago

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@Devansh Jain go to type a solution. u must be on a laptop to upload the image . ull find an the options as latex ,add link etc in the toolbar . the third option is the one to attatch an image . save the image to ur desktop in jpg format and attatch it .

avi solanki - 4 years, 1 month ago

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@Avi Solanki if ur not able to do so .i can share my email adress .i can post the solution on your behalf .

avi solanki - 4 years, 1 month ago

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@Avi Solanki Actually I don't have a laptop so I am sharing the link to my images...There are 2 images Link:1 https://postimg.org/image/r3pqznlk1/ Link:2 https://postimg.org/image/tmk629ntl/

Devansh Jain - 4 years, 1 month ago

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@Devansh Jain ok i will post the solution on your behalf , thanks :)

avi solanki - 4 years, 1 month ago

@Devansh Jain i have posted it

avi solanki - 4 years, 1 month ago

When you say I(n) must be even, you must first prove that it is an integer.It is not beforehand obvious that I(n) is an integer (although it eventually turns out to be an integer).Secondly, the ratio of I(n) and I(n-1) is indeed (-2n).And the value of I(2) is not 8, it is 16 (check your calculations)

Indraneel Mukhopadhyaya - 4 years, 1 month ago

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Kindly check I mentioned that its not 2 n 2n .not 2 n -2n .And if I ( 1 ) I(1) is an integer one could easily build up an intuition that I ( n ) I(n) is gonna be an integer too.And for that I have edited 16 16 .Thanks.

Spandan Senapati - 4 years, 1 month ago

My calculations gave me the ratio -2n.Once you get the ratio, observe that if you recursively multiply the ratio for n=2,3,... , it forms a telescopic product .Hence we get the closed form of the integral = 2 n + 1 ( 1 ) n n ! 2^{n+1}(-1)^{n}n! .

Indraneel Mukhopadhyaya - 4 years, 1 month ago

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