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Calculus Level 1

1 e d x x ( ln x ) 1 / 3 = ? \displaystyle \int_1^e \dfrac{dx}{x(\ln x)^{1/3}} = \ ?


The answer is 1.5.

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6 solutions

Jeel Shah
Mar 1, 2015

For this problem I did decided to use a simple u u substitution of u = ln x u = \ln x .

So, using u = ln x u = \ln x and the change of interval from 0 0 and 1 1 we have:

0 1 d u u 1 3 [ 3 2 u 2 3 ] 0 1 3 2 \begin{aligned} &\int^1_0 \frac{du}{u^{\frac{1}{3}}} \\ &\implies \left[\frac{3}{2}u^{\frac{2}{3}}\right]^1_0 \\ &\implies \boxed{\frac{3}{2}} \end{aligned}

Otto Bretscher
Mar 24, 2015

One should observe that this is an improper integral, undefined at the lower limit of integration, since ln ( 1 ) = 0 \ln(1)=0 . To do the problem properly, use Jeel's method of substitution, u = ln ( x ) u=\ln(x) , to find the integral from 1 + h 1+h to e e , and then let h h approach zero from above.

It's h that was supposed to be u = in ( x ). so 1 + h = e. thus e = h.

Am Kemplin - 1 month, 2 weeks ago
Sarthak Agrawal
Feb 21, 2015

let ln(x)=t^3 so (1/x)dx=3(t^2)dt put it in eq. and solve it.

thank you very much. @Parth Lohomi for so much honour.

Sandeep Bhardwaj - 6 years, 3 months ago

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Thanks sir!

Parth Lohomi - 6 years, 3 months ago

I think Its overrated . It should be a Level 1 Problem. @Calvin Lin

Rajdeep Dhingra - 6 years, 3 months ago

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I think level 2-3 is suitable for this problem. It requires seeing that we can do a suitable U-substitution.

Calvin Lin Staff - 6 years, 3 months ago

1 e d x x ln ( x ) 3 = 3 2 ln 2 3 ( x ) + C Indefinite Integral = 3 2 \begin{aligned} \int_{1}^{e} \frac{dx}{x\sqrt[3]{\ln(x)}} &= \frac{3}{2} \ln^{\frac{2}{3}}(x) \color{grey}{+C} \quad\quad\quad\quad\quad{\text{Indefinite Integral}} \\&= \frac{3}{2} \space \square \end{aligned}

ADIOS!!! \large \text{ADIOS!!!}

Ramez Hindi
Mar 24, 2015

Let u = ln x d u = 1 x d x u=\ln x\Leftrightarrow du=\frac{1}{x}dx and u ( e ) = ln ( e ) = 1 & u ( 1 ) = ln ( 1 ) = 0 u\left( e \right)=\ln \left( e \right)=1\,\,\And \,\,u\left( 1 \right)=\ln \left( 1 \right)=0

So 1 e d x x ( ln x ) 1 / 3 = 0 1 d u u 1 / 3 = 0 1 u 1 / 3 d u = 3 2 [ u 2 / 3 ] 0 1 = 3 2 = 1.5 \int_{1}^{e}{\frac{dx}{x{{\left( \ln x \right)}^{1/3}}}=\int_{0}^{1}{\frac{du}{{{u}^{1/3}}}}=\int_{0}^{1}{{{u}^{-1/3}}du}=\frac{3}{2}\left[ {{u}^{2/3}} \right]_{0}^{1}=\frac{3}{2}=1.5}

Utkarsh Verma
Mar 3, 2015

put Ln(x)=t^3.....change the limits and the solution can be visible easily

😐 so weird.

Am Kemplin - 1 month, 2 weeks ago

I don't know. 😵😳😕😑😐😡😠😴 -_-

Am Kemplin - 1 month, 2 weeks ago

what gives! we have infinity.

Am Kemplin - 1 month, 2 weeks ago

thanks for the solution. 👍

Am Kemplin - 1 month, 2 weeks ago

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