Integration By Parts VS U-Substitution

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Discussion Overview

The discussion centers around the comparison of integration techniques, specifically integration by parts (IBP) and u-substitution, in the context of integrating trigonometric functions. Participants explore when to use each method and discuss alternative approaches.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the choice of integration by parts over u-substitution for integrating the function \(\int \sec^3 x \, dx\), noting a lack of explanation in their review book.
  • Another participant suggests that integration by parts is generally more complicated and typically recommends trying u-substitution first, although acknowledges that obvious choices for u-substitution may not always be present.
  • A participant inquires about other substitution methods beyond u-substitution, leading to a mention of trigonometric substitution and various choices for ordinary substitutions.
  • One participant proposes using u-substitution combined with partial fractions as an alternative to integration by parts, indicating it may involve more work.
  • Tabular integration by parts is introduced as a potentially easier method than classic integration by parts, with a recommendation to learn it.
  • Several participants share resources, including links to articles and videos on tabular integration by parts, highlighting its efficiency in certain cases.
  • It is noted that u-substitution is the simplest method of substitution, and if it is not applicable, alternative methods like integration by parts should be considered.

Areas of Agreement / Disagreement

Participants express differing opinions on the preferred method for integration, with some advocating for integration by parts while others suggest u-substitution or alternative methods. No consensus is reached on a definitive approach for the integral in question.

Contextual Notes

Participants mention various substitution methods and the complexity of integration techniques, but do not resolve the conditions under which each method should be applied. The discussion reflects a range of opinions on the effectiveness of different integration strategies.

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The past few examples in my review book demonstrated u-substitution to integrate trig functions. The example I'm on suddenly shows integration by parts. The book doesn't explain why this method is used over u-sub.

\intsec3x dx

In what situation am I supposed to use one method over the other?
 
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Integration by parts is generally more complicated than ordinary substitution, so I usually try the substitutions first before going to integration by parts. In the integral you show, there aren't any obvious choices for ordinary substitutions, so IBP is called for.
 
Thanks for the reply.
You said "substitutions." What other substitution methods are there other than u-sub?
 
Well, there is trig substitution, but what I meant was that there are often different possibilities for choices for ordinary substitutions.
 
If you don't want to use integration by parts, you could use a u-substitution and partial fractions (but probably more work):
\sec^3x = \frac{1}{\cos^3x} \cdot \frac{\cos x}{\cos x} = \frac{\cos x}{\cos^4x} = \frac{\cos x}{(\cos^2x)^2} = \frac{\cos x}{(1 - \sin^2x)^2}
Let u = sin x, then partial fractions.
 
Tabular Integration by Parts is quite a bit easier to do than the classic IBP.
I recommend you learn it.
 
paulfr, can you provide any resources or links or even an explanation for Tabular IBP? I have never heard of it!
 
I found an interesting pdf on Tabular IBP.
http://www.maa.org/pubs/Calc_articles/ma035.pdf

Tabular IBP is pretty neat and much faster than classic IBP in some cases. Thanks paulfr.
 
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  • #11
U-substitution is the most simple method of substitution. IF you can't do a simple U-substitution and a product is involved, then you want to look at alternative methods, such as integration by parts.
 

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