Simple Integration by Substitution

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Discussion Overview

The discussion revolves around a problem involving integration by substitution, specifically the integral of the function f(x) = x * sqrt(x-1) over the interval [1,2]. Participants are exploring different substitution methods and the implications of providing solutions versus encouraging independent work.

Discussion Character

  • Homework-related
  • Debate/contested
  • Exploratory

Main Points Raised

  • One participant requests help with the integration problem and asks for a visual representation.
  • Another participant offers to guide through the substitution process but questions whether the requester wants assistance or just a completed solution.
  • A participant suggests that the requester may be struggling with algebra and proposes a substitution method, u = sqrt(x-1).
  • Some participants express reservations about the suggested substitution, indicating a preference for different methods.
  • One participant provides a detailed substitution approach, converting the integrand into a polynomial form for integration.
  • Concerns are raised about providing solutions directly, as it may hinder the requester’s learning experience.
  • Another participant suggests using a simpler substitution method but refrains from sharing it to encourage discovery.
  • There is a mention of a previous solution being erased, leading to confusion and frustration among participants.

Areas of Agreement / Disagreement

Participants express differing views on how to approach the integration problem and whether providing solutions directly is beneficial or detrimental to the requester’s learning process. No consensus is reached on the best substitution method.

Contextual Notes

Some participants highlight potential algebraic difficulties and the importance of the requester attempting the problem independently. There is also a mention of a previous solution being removed, which adds to the complexity of the discussion.

adiles
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Please help. I'm having trouble with a simple integration by substitution problem


The integrand is f(x) = x* sqr(x-1)
The interval [1,2]

Please draw it out in a gif file and send it to me via email.

-much appreciated.
 
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If I tell you how to do the substitution will you try to work it out from there? Or do you want someone to just do your homework for you?
 
adiles,

The policy here is that we'll help you if you show what you've tried and where you appear to be stuck.

It seems to me that you may have a problem with algebra (in this case, at least), as the most obvious substitution should get you an easily integrable form.
 
Start with [tex]u=\sqrt{x-1}[/tex]
 
I wouldn't recommend that.
(but I am interested to see how you worked it out with that sub!)
 
Math Is Hard said:
I wouldn't recommend that.
(but I am interested to see how you worked it out with that sub!)

and why not?
::
u=sqrt(x-1)
x=u^2+1
dx=2udu

The integrand becomes (u^2+1)u*2udu
A simple polynomial which has to be integrated from 0 to 1 if i am not mistaken with calculations that are going in my head.
::

-- AI
 
Last edited:
Tenali, I wish you hadn't done that in the thread. This leaves adiles with no work to do.
 
I would use a slightly different (and simpler in my mind) substitution method, but I'll withhold, just to leave a little mystery and hopefully some "pleasure of discovery" for adiles. :smile:
 
since some one already solved it for him, the simplest sub i saw was...

f(x) = x* sqr(x-1)

let
u = x-1
 
Last edited by a moderator:
  • #10
That's why you make the one example something different (but similar enough to demonstrate the point... maybe [itex]\int \sqrt{x-1} \, dx[/itex]) The problem is that all the example in the world usually don't help unless the student actually does a few himself.
 
  • #11
why the hell was my solution erased from this thread?
 
  • #12
Cronxeh I am guessing

Gokul43201 said:
I wish you hadn't done that in the thread. This leaves adiles with no work to do.
 

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