Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    8
    ReputationRep:
    im getting the wrong answer. Can someone tell me what im doing wrong?
    Attached Images
     
    Offline

    9
    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: 11
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

    8
    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

    9
    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

    8
    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

    8
    ReputationRep:
    help
    Offline

    9
    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

    8
    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

    9
    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

    8
    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

    9
    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

    8
    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

    9
    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

    8
    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

    8
    ReputationRep:
    this is the other question.
    Attached Images
     
    • Thread Starter
    Offline

    8
    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

    8
    ReputationRep:
    help?
    Offline

    9
    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

    9
    ReputationRep:
    (Original post by JSM1)
    help?
    Have a bit of patience! There was only four minutes between this and your previous post!
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Did TEF Bronze Award affect your UCAS choices?
    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
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • 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

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