The one with the definite integral in x x !

Calculus Level 4

0 1 x 7 1 ln x d x \large \int_0^1 \dfrac{x^7 -1}{\ln x} \, dx

The integral above has a closed form. Evaluate this closed form.

Give your answer to 2 decimal places.


The answer is 2.08.

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1 solution

Spandan Senapati
Apr 30, 2017

Let's transform the given integral to the form of, I ( a ) = 0 1 x a 1 ln x I(a)=\int_{0}^{1}\frac {x^a-1}{\ln x} with the given limits.The given integral equals ln ( a + 1 ) \ln (a+1) .For this we shall transform the inner term as x a 1 ln x = y = 0 a x y d y ) \frac {x^a-1}{\ln x}=\int_{y=0}^{a}x^ydy) .Now that we have two integrals we may use Fubinis Theorem to switch our integrals.The rest is easy and it yields the required result as ln ( a + 1 ) \ln (a+1) .Now set a = 7 , I = ln 8 = 3 ln 2 = 2.0794 a=7,I=\ln 8=3\ln 2=2.0794 Aliter-Feynmans Trick

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