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

    14
    ReputationRep:


    Can anyone tell me why doing two different methods yields two different answers? Thanks


    Posted from TSR Mobile
    • Study Helper
    Online

    13
    Study Helper
    (Original post by Namige)
    Can anyone tell me why doing two different methods yields two different answers? Thanks
    Because your partial fractions break down is incomplete, and should be:

    \displaystyle\frac{x^2-4x}{(x-2)(x+1)^2}\equiv\frac{A}{x-2}+\frac{B}{(x+1)^2}+\frac{C}{x+  1}
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by ghostwalker)
    Because your partial fractions break down is incomplete, and should be:

    \displaystyle\frac{x^2-4x}{(x-2)(x+1)^2}\equiv\frac{A}{x-2}+\frac{B}{(x+1)^2}+\frac{C}{x+  1}
    Cheers but why was my method wrong? It seems right :/


    Posted from TSR Mobile
    • Study Helper
    Online

    13
    Study Helper
    (Original post by Namige)
    Cheers but why was my method wrong? It seems right :/
    If you plug in only two values, you'll get a solution as there are only two unknowns, when you use just A and B. It's when you try a third value, that the problem shows up.
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by ghostwalker)
    If you plug in only two values, you'll get a solution as there are only two unknowns, when you use just A and B. It's when you try a third value, that the problem shows up.
    So I have used your method an have got this :

    When I plug in x=4, it gives me 3x10^-5 but how can this be, as plugging it into the original formula clearly gives you zero. All other numbers work though...


    Posted from TSR Mobile
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by ghostwalker)
    If you plug in only two values, you'll get a solution as there are only two unknowns, when you use just A and B. It's when you try a third value, that the problem shows up.
    I know, but why is that?


    Posted from TSR Mobile
    • Study Helper
    Online

    13
    Study Helper
    (Original post by Namige)
    When I plug in x=4, it gives me 3x10^-5 but how can this be, as plugging it into the original formula clearly gives you zero. All other numbers work though...
    How on earth do you get 3x1o^-5?

    I get 0.
    • Study Helper
    Online

    13
    Study Helper
    (Original post by Namige)
    I know, but why is that?


    Posted from TSR Mobile
    With just A and B.

    Because your values of A,B are wrong. They can never be right, because there are only two of them.

    If you plug in 2 values for x, you will get values for A and B.

    If you use different values for x, you will get different values for A and B.

    Only when you try 3 values for x, is the problem hightlighted.
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by ghostwalker)
    How on earth do you get 3x1o^-5?

    I get 0.
    For the partial fractions one I get that. Every other number works fine.


    Posted from TSR Mobile
    • Study Helper
    Online

    13
    Study Helper
    (Original post by Namige)
    For the partial fractions one I get that. Every other number works fine.


    Posted from TSR Mobile
    Post your working then.
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by ghostwalker)
    Post your working then.


    This is for x=4.


    Posted from TSR Mobile
    • Study Helper
    Online

    13
    Study Helper
    (Original post by Namige)
    This is for x=4.
    Should have guessed it was a calculator.

    3e-15.

    Well it's a problem with the accuracy of the calculator. It's stored the numbers as decimals, and all it can do is approximate the last digit.
    If you can get it to work in fractions, it should come out correctly.
    • Community Assistant
    Online

    20
    ReputationRep:
    Community Assistant
    (Original post by ghostwalker)
    Should have guessed it was a calculator.

    3e-15.

    Well it's a problem with the accuracy of the calculator. It's stored the numbers as decimals, and all it can do is approximate the last digit.
    If you can get it to work in fractions, it should come out correctly.
    Oh blimey. PRSOM.
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by ghostwalker)
    Should have guessed it was a calculator.

    3e-15.

    Well it's a problem with the accuracy of the calculator. It's stored the numbers as decimals, and all it can do is approximate the last digit.
    If you can get it to work in fractions, it should come out correctly.
    Doing it by Fraction in a single calculation still doesnt work, but by hand does. Very Strange.


    Posted from TSR Mobile
    • Thread Starter
    Offline

    14
    ReputationRep:



    Posted from TSR Mobile
    • Study Helper
    Online

    13
    Study Helper
    (Original post by Namige)
    ...
    Well the problem is the calculator, or how you're using it, and not having one of that model myself, I can't help.

    But the maths, by hand, is correct, 0.
    • Community Assistant
    Online

    20
    ReputationRep:
    Community Assistant
    (Original post by Namige)
    Very Strange.
    "Never trust a machine you can carry." Dusty Rivers
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by ghostwalker)
    Well the problem is the calculator, or how you're using it, and not having one of that model myself, I can't help.

    But the maths, by hand, is correct, 0.
    Thank you for your help
 
 
 
  • 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
    Would you like to hibernate through the winter months?
    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.