Logarithm powers! (13)

Calculus Level 5

0 1 x 2 ( ln ( 1 x ) ) 3 d x = ? \large \int_0^1 x^2 \left(\ln \left(\frac{1}{x} \right) \right)^3 \, dx = \, ?

  • The integral above has a closed form. Find the value of this closed form.

  • Give your answer to 3 3 decimal places.


The answer is 0.074074.

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2 solutions

0 1 x 2 ( ln ( 1 x ) ) 3 d x \large \displaystyle \int_0^1 x^2 \, \left(\ln \left(\frac{1}{x} \right) \right)^3 \, dx

= 0 1 x 2 ( ln ( x ) ) d x \large \displaystyle = - \int_0^1 x^2 \left(\ln (x) \right) \, dx

Let x = e y d x = e y d y \large \displaystyle \color{#EC7300}{\text{Let } x = e^{-y}} \implies \color{royalblue}{dx = - e^{-y} \, dy}

= 0 e 3 y y 3 d y \large \displaystyle = \int_0^{\infty} e^{-3y} \, y^3 \, dy

Again Let 3 y = u 3 d y = d u \large \displaystyle \color{#69047E}{\text{Again Let } 3y = u} \implies \color{#BA33D6}{3 \, dy = du}

= 0 u 3 3 3 × e u 3 d u \large \displaystyle = \int_0^{\infty} \frac{u^3}{3^3} \times \frac{e^{-u}}{3} \, du

= 1 3 4 Γ ( 4 ) 0.074074 \large \displaystyle = \color{#D61F06}{\frac{1}{3^4}} \, \color{#3D99F6}{\Gamma (4)} \approx \color{#20A900}{\boxed{0.074074}}

There's a typo @Samara Simha Reddy , In the last 2nd line change u u to u 3 u^3

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Oh. Yeah! Thank You!

Samara Simha Reddy - 5 years ago

XD isnt it the same question?

Ashish Menon - 5 years ago

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It is! I deleted the previous question.

Samara Simha Reddy - 5 years ago

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This question is tricky. Nice solution (+1)

Ashish Menon - 5 years ago

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@Ashish Menon Thank You!

Samara Simha Reddy - 5 years ago

J ( a ) = 0 1 x a d x = 1 a + 1 \displaystyle J(a)=\int_{0}^{1}x^a dx = \frac{1}{a+1}

J ( 2 ) = 0 1 x 2 ln 3 x d x = 6 81 \displaystyle -J'''(2) = -\int_{0}^{1} x^2 \ln^3 x dx = -\frac{6}{81} , So 0 1 x 2 ln 3 ( 1 x ) d x = 6 81 0.074 \displaystyle \int_{0}^{1} x^2 \ln^3(\frac{1}{x}) dx = \frac{6}{81}\approx 0.074

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