Analyzing the Definite Integral (1+x)^(-1/2)

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Discussion Overview

The discussion revolves around the analytic solution to the definite integral \(\int_{0}^{\infty}{\frac{dx}{(1+x)\sqrt{x}}}\). Participants explore various approaches to solving this integral, including substitutions and the application of limits.

Discussion Character

  • Exploratory
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about the existence of an analytic solution to the integral, noting it has been solved numerically.
  • Another participant suggests the substitution \(u^2 = x\) as a potential method for solving the integral.
  • A subsequent post reiterates the substitution and arrives at the expression \(2\int\frac{1}{1+u^2}du\), indicating uncertainty about the next steps involving trigonometric substitutions.
  • Another participant asserts that the solution involves the arc tangent function.
  • One participant acknowledges the arc tangent solution but expresses concern that their derived solution contains a variable, contrasting it with a reference solution of \(\pi\) from a mathematical table.
  • Another participant points out the distinction between indefinite and definite integrals, suggesting that the limits of integration need to be applied to obtain the correct result.
  • A later reply confirms understanding of how the solution \(\pi\) is derived when applying the integration limits.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the solution process, as there are multiple approaches and some uncertainty regarding the application of limits and the nature of the integral.

Contextual Notes

There is a lack of clarity regarding the transition from the indefinite integral to the definite integral, and the discussion does not resolve the assumptions or steps involved in the integration process.

SW VandeCarr
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Is there an analytic solution to the following integral? (Not a homework question, solved numerically).

[tex]\int_{0}^{\infty}{\frac{dx}{(1+x)\sqrt{x}}=\pi[/tex]
 
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Let u^2 = x
 
l'Hôpital said:
Let u^2 = x

OK I get: [tex]2\int\frac{1}{1+u^2}du[/tex]

I'm a bit rusty at this. I think I need to make some trig substitutions here, but I'm at a loss as to exactly what.
 
Last edited:
It's just there, the arc tangent.
 
jrlaguna said:
It's just there, the arc tangent.

Right. I finally saw that. However, the solution: [tex]2\arctan(\sqrt{x})+C[/tex] contains a variable. The solution given in the CRC Standard Mathematical Tables 14th ed, page 342, Eq 489 is [tex]\pi[/tex].

EDIT:Yes, I know it's old, but I doubt a new solution has been invented. I bought it for $2.
 
Last edited:
You looked up the indefinite integral. You have a definite integral. Apply the integration limits.
 
D H said:
You looked up the indefinite integral. You have a definite integral. Apply the integration limits.

The form of the integral in the book is just as I wrote it in the first post, but I see how the solution [tex]\pi[/tex] is obtained. Thanks.
 

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