Turn on thread page Beta
    • Thread Starter
    Offline

    2
    ReputationRep:
    My brain isn't working properly today, and while revising c4 I managed to confuse myself.

    Why can't you integrate for example sin^2 (x) using inverse chain rule, ie increase power by 1, divide by new power, divide by differential of sinx to get (tanx sin^2 x)/3 ?
    Offline

    18
    ReputationRep:
    What's the inverse chain rule?
    • Thread Starter
    Offline

    2
    ReputationRep:
    Well actually ye it's not in the OCR book but it's how my teacher describes it. Like if you were differentiating (2x)^2 you could multiply by power, reduce power by 1, multiply by differential of whats in the brackets getting 2x2x(2x) = 8x

    So doing it the other way around basically, like if I was integrating (2x)^2 I could increase the power by 1, divide by the new power, divide by differential of whats in the brackets ie (2x)^3 / (3 x 2) = 4x^3 / 3 +k or w/e but ye.
    Offline

    15
    ReputationRep:
    (Original post by SimonM)
    What's the inverse chain rule?
    I think that's what they call integration by substitution :confused:
    Offline

    12
    ReputationRep:
    (Original post by jamie092)
    My brain isn't working properly today, and while revising c4 I managed to confuse myself.

    Why can't you integrate for example sin^2 (x) using inverse chain rule, ie increase power by 1, divide by new power, divide by differential of sinx to get (tanx sin^2 x)/3 ?
    You can't start dividing by functions of x, you can only adjust numerical factors. You can also check if you're wrong. Try differentiating what you think you'd get. You don't get back to what you want.
    Offline

    0
    ReputationRep:
    Call it reverse chain, not inverse
    Offline

    12
    ReputationRep:
    (Original post by boromir9111)
    I think that's what they call integration by substitution :confused:
    I think he might mean by "by inspection." But don't hold me to it.
    Offline

    15
    ReputationRep:
    (Original post by dknt)
    I think he might mean by "by inspection." But don't hold me to it.
    Ahh yes, there was a thread on this a while ago, didn't end well lol but inspection is a form of substitution.....loonggg!
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by dknt)
    You can't start dividing by functions of x, you can only adjust numerical factors. You can also check if you're wrong. Try differentiating what you think you'd get. You don't get back to what you want.
    Hmm so you can't divide by functions of x when you integrate but you can multiply by functions of x when you differentiate? weird; P
    Offline

    12
    ReputationRep:
    (Original post by jamie092)
    Hmm so you can't divide by functions of x when you integrate but you can multiply by functions of x when you differentiate? weird; P
    What?

    What I mean is you can only adjust numerical factors. When you integrate, sometimes you can do it by inspection and use a little bit of prior knowledge to "guess" the derivative. You then check by differentiating and then adjust the numerical factor. For example, if you differentiate and end up with a 2 in front, but you need a 3, you adjust the numerical factor to (3/2) to give you this. But if you end up with an extra "x" term, which you don't have in your integral, you can't put a (1/x) in front of it.
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by dknt)
    What?

    What I mean is you can only adjust numerical factors. When you integrate, sometimes you can do it by inspection and use a little bit of prior knowledge to "guess" the derivative. You then check by differentiating and then adjust the numerical factor. For example, if you differentiate and end up with a 2 in front, but you need a 3, you adjust the numerical factor to (3/2) to give you this. But if you end up with an extra "x" term, which you don't have in your integral, you can't put a (1/x) in front of it.
    Well ye I know that I suppose it's just something I'm going to have to ask my teacher since I can't really explain what I mean here ;P.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: March 20, 2011

University open days

  1. University of Bradford
    University-wide Postgraduate
    Wed, 25 Jul '18
  2. University of Buckingham
    Psychology Taster Tutorial Undergraduate
    Wed, 25 Jul '18
  3. Bournemouth University
    Clearing Campus Visit Undergraduate
    Wed, 1 Aug '18
Poll
How are you feeling in the run-up to Results Day 2018?
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

Equations

How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.