Turn on thread page Beta
    • Thread Starter
    Offline

    13
    ReputationRep:
    im getting the wrong answer. Can someone tell me what im doing wrong?
    Attached Images
     
    Offline

    12
    ReputationRep:
    (Original post by JSM1)
    im getting the wrong answer. Can someone tell me what im doing wrong?
    The error is in your fourth line, when you factorise out (2x + 1)^(-2).
    Offline

    2
    ReputationRep:
    Name:  IMG_9240.jpg
Views: 26
Size:  496.5 KB
    I've just started this topic in school too not sure if I'm right did it without a calculator lol
    (Original post by JSM1)
    im getting the wrong answer. Can someone tell me what im doing wrong?
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by Pangol)
    The error is in your fourth line, when you factorise out (2x + 1)^(-2).
    what have i done wrong? I used the exact method in previous questions where you expand
    Offline

    12
    ReputationRep:
    (Original post by JSM1)
    what have i done wrong? I used the exact method in previous questions where you expand
    You are factorising something that has the form A x^(-2) + B x^(-1). If you factorised x^(-2) out of this, what would you have?

    (It's not a huge error, but one missing symbol has lead you to head off in the wrong direction.)
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by Pangol)
    You are factorising something that has the form A x^(-2) + B x^(-1). If you factorised x^(-2) out of this, what would you have?

    (It's not a huge error, but one missing symbol has lead you to head off in the wrong direction.)
    do you add the terms?
    • Thread Starter
    Offline

    13
    ReputationRep:
    help
    Offline

    12
    ReputationRep:
    (Original post by JSM1)
    do you add the terms?
    Sorry - thought I had replied to this. I think you are right, but what do you mean by "the terms"? They change their form after factorising.
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by Pangol)
    Sorry - thought I had replied to this. I think you are right, but what do you mean by "the terms"? They change their form after factorising.
    do you add over here
    Attached Images
     
    Offline

    12
    ReputationRep:
    (Original post by JSM1)
    do you add over here
    Yeah, that's the bit that is wrong. You should be adding the (-2x + 2) and the (2x + 1), not multiplying them - but even if you were supposed to be multiplying them (which you are not!), there should be a bracket around the first part.
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by Pangol)
    Yeah, that's the bit that is wrong. You should be adding the (-2x + 2) and the (2x + 1), not multiplying them - but even if you were supposed to be multiplying them (which you are not!), there should be a bracket around the first part.
    do you know the reason why you add these terms as in previous questions i expanded the bracket by the number in front of it.
    Offline

    12
    ReputationRep:
    (Original post by JSM1)
    do you know the reason why you add these terms as in previous questions i expanded the bracket by the number in front of it.
    I can't be sure without seeing the question, but I expect you got the right answer by coincidence.

    This is just factorisation. In the example I mentioned above, A x^(-2) + B x^(-1), we can factorise out x^(-2) from both terms. How many of these do we have? In the first term, we have A of them. In the second term, we have Bx of them. So altogether, we have A + Bx of them, that is, A x^(-2) + B x^(-1) = x^(-2) [A + Bx].
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by Pangol)
    I can't be sure without seeing the question, but I expect you got the right answer by coincidence.

    This is just factorisation. In the example I mentioned above, A x^(-2) + B x^(-1), we can factorise out x^(-2) from both terms. How many of these do we have? In the first term, we have A of them. In the second term, we have Bx of them. So altogether, we have A + Bx of them, that is, A x^(-2) + B x^(-1) = x^(-2) [A + Bx].
    Question:

    Find the value of dy/dx at the point x=2 on the curve with the equation

    y=(x-1)(2x+1)^-1
    Offline

    12
    ReputationRep:
    (Original post by JSM1)
    Question:

    Find the value of dy/dx at the point x=2 on the curve with the equation

    y=(x-1)(2x+1)^-1
    Yeah, well to check this I'd need to do the whole thing! I'd rather see your working. But more to the point - do you get what I am saying about factorisation?
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by Pangol)
    Yeah, well to check this I'd need to do the whole thing! I'd rather see your working. But more to the point - do you get what I am saying about factorisation?
    yes i do understand abit but previous questions have the same format so why do you multiply for those
    • Thread Starter
    Offline

    13
    ReputationRep:
    this is the other question.
    Attached Images
     
    • Thread Starter
    Offline

    13
    ReputationRep:
    over here i have multiplied out 1(2x+1) which gave me 2x+1,

    then adding 1x.. 2x+1+1x gave me........... 3x+1
    Attached Images
     
    • Thread Starter
    Offline

    13
    ReputationRep:
    help?
    Offline

    12
    ReputationRep:
    (Original post by JSM1)
    over here i have multiplied out 1(2x+1) which gave me 2x+1,

    then adding 1x.. 2x+1+1x gave me........... 3x+1
    This is right. When you factorise out the 3(2x + 1)^(-1/2), you are left with x from the first term, and 2x + 1 from the second term. You then correctly add these together.

    In the problem question, you are left with -2x + 2 from the first term, and 2x + 1 from the second term. You should be adding these together. What you have done is a strange mixture of taking the -2x from the first term, and multiplying the second term by the 2 from the first term.
    Offline

    12
    ReputationRep:
    (Original post by JSM1)
    help?
    Have a bit of patience! There was only four minutes between this and your previous post!
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: October 16, 2017

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.