Nice Integral

Calculus Level 4

1 0 x 2 + 2 x ln ( x + 1 ) d x \large \int_{-1}^0 \dfrac{x^2+2x}{\ln(x+1)} \, dx

The integral above has a simple closed form. Find this closed form.

Give your answer to 3 decimal places.


The answer is 1.0986.

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1 solution

Jatin Yadav
Mar 26, 2014

Put x + 1 = t x+1 = t to get the integral as :

0 1 x 2 1 ln ( x ) d x \displaystyle \int_{0}^{1} \frac{x^2-1}{\ln(x)} dx

Let I ( a ) = 0 1 x a 1 ln ( x ) d x I(a) = \int_{0}^{1} \frac{x^a-1}{\ln(x)} dx

I ( a ) = 0 1 x a ln ( x ) ln ( x ) d x = 1 a + 1 \Rightarrow I'(a) = \int_{0}^{1} \frac{x^a \ln(x)}{\ln(x)} dx = \frac{1}{a+1}

I ( a ) = ln ( a + 1 ) + c \Rightarrow I(a) = \ln(a+1) + c

Also, I ( 0 ) = 0 1 1 1 ln ( x ) d x = 0 I(0) = \displaystyle \int_{0}^{1} \frac{1-1}{\ln(x)} dx = 0

Hence, I ( a ) = ln ( a + 1 ) I(a) = \ln(a+1)

Hence, our answer is ln ( 2 + 1 ) = ln ( 3 ) = 1.09861 \ln(2+1) = \ln(3) = 1.09861

Well done. btw, congrats on 500 upvotes (im still on 476)

Anish Puthuraya - 7 years, 2 months ago

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Thanks!

jatin yadav - 7 years, 2 months ago

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btw, how do you know when to use the "derivative method" to find an integral?

Anish Puthuraya - 7 years, 2 months ago

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@Anish Puthuraya Sorry, I don't know that. But you can keep that as a way to be applied at last.

jatin yadav - 7 years, 2 months ago

@Anish Puthuraya This might help.

Vijay Raghavan - 7 years, 2 months ago

@jatin yadav can you share the link of the question here , i wrote but did'nt founded it , i too want to upvote it

sandeep Rathod - 6 years, 6 months ago

Brilliant!

Milly Choochoo - 7 years, 2 months ago

That is commendable!

Vijay Raghavan - 7 years, 2 months ago

that was easy with feymann calculus.! :)

Pradeep Ch - 7 years, 2 months ago

you are a genius, man!!

Anup Kumar - 6 years, 11 months ago

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