Integral of xln(x) with 0 as a limit of integration

In summary, the conversation discusses the integral \displaystyle{\int_{0}^{1}}x\ln{x}\,dx=-\frac{1}{4} and the question of why this result is obtained by Wolfram Alpha. The conversation then goes on to explain that taking the limit as \epsilon \to 0 justifies this result, as it is the definition of an indefinite integral with a singularity at one end of the interval of integration. Continuity is also mentioned as a possible justification.
  • #1
fixed_point
2
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Homework Statement


Not exactly a homework problem, but I noticed that wolfram alpha gives
[itex]\displaystyle{\int_{0}^{1}}x\ln{x}\,dx=-\frac{1}{4}[/itex]
I was wondering why this is so.

2. Homework Equations & solution attempt
For [itex]\epsilon>0[/itex]
[itex]\displaystyle{\int_{\epsilon}^{1}}x\ln{x}\,dx=-\frac{1}{4}-\left(\frac{1}{4}\epsilon^2(2\ln{\epsilon}-1)\right)[/itex]
Taking [itex]\epsilon \to 0[/itex] gives the answer provided by wolfram alpha.

My question is what justifies this last step of taking [itex]\epsilon \to 0[/itex]? Continuity?
 
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  • #2
It's the definition of an indefinite integral with a singularity at one end of the interval of integration.
 
  • #3
Dick said:
It's the definition of an indefinite integral with a singularity at one end of the interval of integration.
Thank you for your help.
 

FAQ: Integral of xln(x) with 0 as a limit of integration

What is the formula for the integral of xln(x) with 0 as a limit of integration?

The formula for the integral of xln(x) with 0 as a limit of integration is ∫xln(x)dx = x(ln(x) - 1) + C.

Can the integral of xln(x) with 0 as a limit of integration be solved using substitution?

Yes, the integral of xln(x) with 0 as a limit of integration can be solved using substitution. Let u = ln(x) and du = 1/x dx, then the integral becomes ∫xln(x)dx = ∫u du = u^2/2 + C = (ln(x))^2/2 + C.

What is the value of the integral of xln(x) with 0 as a limit of integration?

The value of the integral of xln(x) with 0 as a limit of integration is undefined, as ln(x) is not defined at x=0. However, the limit of the integral as x approaches 0 from the right is 0.

Can the integral of xln(x) with 0 as a limit of integration be evaluated using integration by parts?

Yes, the integral of xln(x) with 0 as a limit of integration can be evaluated using integration by parts. Let u = ln(x) and dv = x dx, then du = 1/x dx and v = x^2/2, and the integral becomes ∫xln(x)dx = x^2/2 ln(x) - ∫x/2 dx = x^2/2 ln(x) - x/4 + C.

What is the graph of the integral of xln(x) with 0 as a limit of integration?

The graph of the integral of xln(x) with 0 as a limit of integration is a curve that approaches infinity as x approaches 0 from the right. It also has a vertical asymptote at x=0 and is symmetrical about the y-axis.

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