U-sub integral: Not sure where i went wrong.

  • Thread starter Thread starter dwdoyle8854
  • Start date Start date
  • Tags Tags
    Integral
Click For Summary

Homework Help Overview

The discussion revolves around evaluating an integral using the method of substitution. The original poster expresses confusion over obtaining two different answers from two approaches to the same integral, questioning the validity of their operations and the correctness of the textbook's answer.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to evaluate the integral in two different ways and is unsure where the discrepancy arises. Some participants question the algebraic manipulations involved in the evaluations, suggesting that there may be errors in the simplification of rational expressions.

Discussion Status

The discussion is ongoing, with participants providing feedback on the algebraic steps taken by the original poster. There is an indication that some guidance has been offered regarding the algebraic errors, but no consensus has been reached on the correct approach or resolution of the problem.

Contextual Notes

The original poster has provided images of their work, which are central to the discussion, but the exact details of the integral and the specific operations performed are not fully outlined in the posts.

dwdoyle8854
Messages
16
Reaction score
0

Homework Statement



Im not sure what i did wrong, every operation i took to evaluate the integral seems valid. I took pictures of the integral evaluated two different ways, producing two different answers. What bugs me is even if i evaluate (as i did) from say 0 to 1 the numbers are different, so they are not equivalent. I don't see any errors in the first one, but the textbook lists the latter as a correct answer.

The incorrect way:
2QfhdVs.jpg

Correct:
24E2jat.jpg

Homework Equations





The Attempt at a Solution


The incorrect way:
<a href="[PLAIN]http://imgur.com/2QfhdVs"><img src="http://i.imgur.com/2QfhdVs.jpg?1" title="Hosted by imgur.com" alt="" /></a>[/PLAIN]

Correct:
<a href="[PLAIN]http://imgur.com/24E2jat"><img src="http://i.imgur.com/24E2jat.jpg" title="Hosted by imgur.com" alt="" /></a>[/PLAIN]
 
Last edited by a moderator:
Physics news on Phys.org
Saying u/(2+u^2-2u) equals u/2+u^2/u-2u/u is seriously bad algebra. (a+b)/c=a/c+b/c. c/(a+b) IS NOT equal to c/a+c/b. Try some numbers.
 
dwdoyle8854 said:

Homework Statement



I'm not sure what i did wrong, every operation i took to evaluate the integral seems valid. I took pictures of the integral evaluated two different ways, producing two different answers. What bugs me is even if i evaluate (as i did) from say 0 to 1 the numbers are different, so they are not equivalent. I don't see any errors in the first one, but the textbook lists the latter as a correct answer.

The incorrect way:
[ IMG]http://i.imgur.com/2QfhdVs.jpg?1[/PLAIN]
Correct:
[ IMG]http://i.imgur.com/24E2jat.jpg[/PLAIN]

Homework Equations



The Attempt at a Solution


The incorrect way:
<a href="[PLAIN]http://imgur.com/2QfhdVs"><img src="http://i.imgur.com/2QfhdVs.jpg?1" title="Hosted by imgur.com" alt="" /></a>[/PLAIN]

Correct:
<a href="[PLAIN]http://imgur.com/24E2jat"><img src="http://i.imgur.com/24E2jat.jpg" title="Hosted by imgur.com" alt="" /></a>[/PLAIN]
Your error is in doing basic algebra with a rational expression.

\displaystyle \frac{u}{u^2-2u+2}\ne\frac{u}{u^2}-\frac{u}{2u}+\frac{u}{2}

After all, is \displaystyle \ \ \frac{1}{1+2+3}=\frac{1}{1}+\frac{1}{2}+\frac{1}{3}\ ? \ \
 
Last edited by a moderator:
oh christ. I am dumb.

Apologies.
 

Similar threads

  • · Replies 2 ·
Replies
2
Views
3K
Replies
18
Views
5K
Replies
1
Views
2K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 14 ·
Replies
14
Views
3K
  • · Replies 1 ·
Replies
1
Views
3K
Replies
21
Views
3K
Replies
1
Views
2K