Indefinite Integration of a Rational Expression

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Homework Help Overview

The discussion revolves around the indefinite integration of a rational expression involving the integral of the function \((r^2 - 2r) / (r^3 - 3r^2 + 1)\) with respect to \(r\). Participants are exploring the process of substitution in integration.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the transformation of the numerator and its relation to the denominator, particularly questioning how the numerator can be simplified or interpreted in the context of substitution. There is also a focus on the importance of including the differential part in substitution.

Discussion Status

The discussion has progressed with some participants clarifying concepts related to substitution and the relationship between the numerator and denominator. There appears to be a growing understanding among participants, although not all aspects have been fully resolved.

Contextual Notes

Participants are navigating through the specifics of the integration process, with some confusion regarding the notation and the steps involved in substitution. The original poster expresses uncertainty about the manipulation of the numerator.

communitycoll
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Homework Statement


<Indefinite integral sign here>[r^2 -2r] / [r^3 - 3r^2 + 1]dr
or the second example in the "Substitution" section here:
http://people.clarkson.edu/~sfulton/ma132/parfrac.pdf

Homework Equations


nada.


The Attempt at a Solution


Nothing to really attempt, I just don't get what they do with the numerator (i.e., how it turns into a 1).
 
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hi communitycoll! :smile:
(have an integral: ∫ and try using the X2 button just above the Reply box :wink:)
communitycoll said:
… I just don't get what they do with the numerator (i.e., how it turns into a 1).

ah, nooo, the numerator isn't r2 - 2r, it's (r2 - 2r)dr !

(and of course that's 1 times dw :wink:)

the trick in substitution is that you always have to substitute the "d" part also! :smile:
 
If you read that attachment carefully, you would have seen this:
Solution: Here we notice that the numerator is the derivative of the denominator (to within a constant factor).
So they let
w = r3 - 3r2 + 1,
so
dw = 3r2 - 6r dr,
which is the same as
dw = 3(r2 - 2r) dr.

Do you see it now?
 
Okay then. Yeah, I understand now. Thanks.
 

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