How do I integrate ln(4+y^2)dy at the end of a double integral?

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Discussion Overview

The discussion revolves around the integration of the function ln(4+y^2) within the context of a double integral, specifically focusing on the steps required to evaluate this integral after determining the bounds for x and y. The scope includes mathematical reasoning and integration techniques.

Discussion Character

  • Mathematical reasoning
  • Homework-related
  • Technical explanation

Main Points Raised

  • One participant expresses difficulty in integrating ln(4+y^2) after completing a double integral involving the function x/(x^2+y^2).
  • Another participant inquires about the bounds for x and y, which are later specified as the region R = [1,2] * [0,1].
  • Several participants suggest different methods for integrating ln(4+y^2), including standard techniques for logarithmic integrals and substitution methods.
  • One participant proposes using integration by parts followed by a trigonometric substitution as a potential approach.
  • Another participant argues that substitution is unnecessary and suggests performing integration by parts directly, followed by a strategic manipulation of the remaining integral.
  • A participant confirms the integral being discussed and provides a specific expression for it, indicating a successful application of integration by parts.
  • Another participant suggests splitting the integral into separate parts for easier evaluation, proposing a substitution for ln(1+y^2) as an example.
  • One participant mentions that they have already solved the integral and shares their approach using integration by parts directly.

Areas of Agreement / Disagreement

Participants present multiple approaches to the integration problem, with no consensus on a single method being preferred. Some methods are debated, and various opinions on the necessity of substitution and integration techniques are expressed.

Contextual Notes

Participants do not explicitly resolve the mathematical steps or assumptions involved in the integration process, leaving some aspects of the discussion open to interpretation and further exploration.

Who May Find This Useful

Readers interested in integration techniques, particularly those involving logarithmic functions, and those seeking assistance with double integrals may find this discussion relevant.

dave_western
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Stuck at the end of a double integral, still have to integrate ln(4+y^2)dy

Assuming I did the right first step. Original double integral is

x/(x^2+y^2)

Thanks!
 
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What are the bounds on x and y?
 
the region R = [1,2] * [0,1]
 
Have you had any thoughts on integrating that? I see two obvious things to try:

(1) Do what you normally do with integrals of logarithms.
(2) Make a substitution.
 
eg. Integration by parts then trig sub.
 
There's no need for substitution for that integral. Part integration once then a smart move in the numerator of the remaining integral and it's done.

Daniel.
 
is the integral we are talking about \int_{0}^{1}\int_{1}^{2}\frac{x}{x^2+y^2}dxdy?

i get \frac{1}{2}\int_{0}^{1}\left[\ln(4+y^2)-\ln(1+y^2)\right]dy
what should i do next? (edit: i got it. integration by parts.)
 
Last edited:
Murshid, split the integral up, do them separately perhaps? Eg, say for \int ln(1+y^2) dy we could let y^2 equal u. Find du/dy, easy. Then, solve for dy. Now substitute that value in. We end up with \int \frac{ln (u+1)}{2(u)^{1/2}} du. Then some nice integration by parts and we are done?

Takes a while though, I hope your patient.
 
well i got it already. thanks anyway.
but we can directly use integration by parts on this \int ln(1+y^2) dy by letting u = \ln(1+y^2) and dv = dy
 

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