Integrating Fractions with Substitution

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Discussion Overview

The discussion revolves around the integration of the function ∫(6x+5)/(2x+1)dx, specifically exploring the use of substitution as a method for solving the integral. Participants examine the validity of their approaches and the interpretation of results, including the comparison with computational tools like Wolfram Alpha.

Discussion Character

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

Main Points Raised

  • One participant describes their approach using substitution, defining u = 2x+1 and deriving an integral that leads to a result involving a constant term.
  • Another participant argues that the difference in results is negligible since the constant term (3/2) can be absorbed into the constant of integration (C).
  • A third participant expresses the challenge of verifying calculus work due to the variability in answer forms, especially when using computational tools.
  • One participant suggests a method of verifying integration by differentiating the result to check if it returns to the original function, noting that different forms can arise from various substitution methods.

Areas of Agreement / Disagreement

Participants generally agree on the validity of the substitution method and the interpretation of the constant term, but there remains some uncertainty regarding the comparison of their results with those from computational tools. The discussion does not reach a consensus on the implications of these differences.

Contextual Notes

Participants acknowledge that different forms of answers can arise from integration techniques and that computational tools may simplify results differently. There is an implicit recognition of the limitations in verifying answers due to these variations.

Who May Find This Useful

This discussion may be useful for students and practitioners of calculus, particularly those interested in integration techniques and the nuances of verifying their results against computational outputs.

cp255
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So the problem is ∫(6x+5)/(2x+1)dx. I know the proper way to solve this is to long divide these two expressions and then solve. However, I tried doing it with substitution.

u = 2x+1
dx = du/2
I then reasoned that 3u + 2 = 6x+5 since 3(2x+1) + 2 = 6x+3+2 = 6x+5 so I substituted it on top.
(1/2)∫(3u+2)/u du
Solving this integral gives me (3/2)u + ln|u| + C which equals (6x+3)/2 + ln|2x+1| + C.

However, Wolfram Alpha says the answer is 3x + ln|2x+1| + C. I don't understand what I did wrong or why I can't do what I did.
 
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It's the same answer. The 3/2 term gets absorbed into the + C term because 3/2 is just a constant. You did the problem correctly.
 
Well that makes sence. It is sometimes hard to check my calculus work especially with computers since there are so many different forms an answer can take.
 
A useful trick I do for integration problems is plug the answer in and take the derivative. If you get the original problem back, you have the correct answer. Wolfram doesn't always compute it the same way as you and might simplify it differently. This is especially apparent if you do a trig substitution and have a theta hanging around in your answer. Using the triangle you made for the trig substitution, you can have 6 different answers for the value of theta that are the same (arcsin, arccos, etc).

You can also differentiate by hand if the answer isn't too complicated to check yourself.
 

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