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

    2
    ReputationRep:
    Hey this is the question and this is what I've done so far:

    Complete the square and find solution to x.

    - 2x^2 - 9x - 7 = 0

    - x^2 - 4.5x = 3.5

    - (x-2.25)^2 + 5.0625 = 3.5

    - (x-2.25)^2 = - 1.5625

    - How do I square root the negative (even though you can't) - without using imaginary numbers, which I know nothing about yet.

    I need two values for X ? Where the hell am I going wrong. Hopefully just a lil mistake.
    Offline

    9
    ReputationRep:
    (Original post by Tweaky)
    Hey this is the question and this is what I've done so far:

    Complete the square and find solution to x.

    - 2x^2 - 9x - 7 = 0

    - x^2 - 4.5x = 3.5

    - (x-2.25)^2 + 5.0625 = 3.5

    - (x-2.25)^2 = - 1.5625

    - How do I square root the negative (even though you can't) - without using imaginary numbers, which I know nothing about yet.

    I need two values for X ? Where the hell am I going wrong. Hopefully just a lil mistake.
    It should be (x-2.25)^2 - 5.0625 = 3.5.

    You have written it as +ve.
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by raheem94)
    It should be (x-2.25)^2 - 5.0625 = 3.5.

    You have written it as +ve.

    No wait...

    (x-2.25)(x-2.25) = x^2 -4.5x then (-2.25 x -2.25 is positive) So it is positive...?
    Offline

    14
    ReputationRep:
    (Original post by Tweaky)
    Hey this is the question and this is what I've done so far:

    Complete the square and find solution to x.

    - 2x^2 - 9x - 7 = 0
    The obvious first thing to do, is to multiply throughout by minus one.
    Offline

    9
    ReputationRep:
    (Original post by steve2005)
    The obvious first thing to do, is to multiply throughout by minus one.
    It isn't a negative sign, he is just putting them in front of all his lines.
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by steve2005)
    The obvious first thing to do, is to multiply throughout by minus one.
    its not " -2x^2"

    The - was a bullet point. If that's not what you meant, could you elaborate because I don't understand. Cheers man
    Offline

    16
    ReputationRep:
    (Original post by Tweaky)
    its not " -2x^2"

    The - was a bullet point. If that's not what you meant, could you elaborate because I don't understand. Cheers man
    Poor choice of "bullet"
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by TenOfThem)
    Poor choice of "bullet"
    Great, but now you know could you help me please xx
    Offline

    9
    ReputationRep:
    (Original post by Tweaky)
    No wait...

    (x-2.25)(x-2.25) = x^2 -4.5x then (-2.25 x -2.25 is positive) So it is positive...?
    x^2 - 4.5x = 3.5

     \displaystyle x^2 - \frac92 x = \frac72 \implies \left(x- \frac94 \right)^2 - \left( \frac94 \right)^2 = \frac72

     \displaystyle  \left(x- \frac94 \right)^2 = x^2 - \frac92 x + \left( \frac94 \right)^2

    So it should be negative,

    Do you get it?
    Offline

    16
    ReputationRep:
    (Original post by Tweaky)
    Great, but now you know could you help me please xx
    No need for me to do so

    Raheem has told you where you went wrong but you chose to argue with him
    Offline

    14
    ReputationRep:
    (Original post by Tweaky)
    .
    You want x^2 - 4.5x on the LHS. So this is the same as writing (x-2.25)^2 + an adjustment. (x-2.25)^2 = x^ - 4.5x + 5.0625 so our 'adjustment' must be -5.0625 to cancel it out.
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by TenOfThem)
    No need for me to do so

    Raheem has told you where you went wrong but you chose to argue with him
    I just didn't understand.
    Offline

    15
    ReputationRep:
    2x^2 - 9x - 7 = 0
    x^2 - (9/2)x - (7/2) = 0
    (x - 9/4)^2 - (9/4)^2 - (7/2) = 0
    (x - 9/4)^2 - 81/16 - 7/2 = 0
    (x - 9/4)^2 - 81/16 - 55/16 = 0
    (x - 9/4)^2 - 136/16 = 0
    (x - 9/4)^2 - 17/2 = 0
    (x - 9/4)^2 = 17/2

    Can you go from here?

    PS. Use fractions where you can. It's better maths.
    Offline

    16
    ReputationRep:
    (Original post by Tweaky)
    I just didn't understand.
    Then I suggest that you go and look at your notes on "completing the square" as you seem to have a basic mis-understanding of the process
    Offline

    9
    ReputationRep:
    (Original post by Tweaky)
    I just didn't understand.
    Go to this link to learn this.
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by hassi94)
    You want x^2 - 4.5x on the LHS. So this is the same as writing (x-2.25)^2 + an adjustment. (x-2.25)^2 = x^ - 4.5x + 5.0625 so our 'adjustment' must be -5.0625 to cancel it out.
    Sweet, yeah I get it now. Was a small error, cheers. And thanks Raheem. To the other two users, arrogance won't get you far in life.
    Offline

    16
    ReputationRep:
    (Original post by Tweaky)

    I didn't realize it was a crime to make a mistake and not understand someone straight away.
    I simply pointed out the bullet was a bad choice

    Your response was not polite ... and I pointed out that you already had the answer


    I think you need to read responses to the question, think about them, and absorb the maths advice instead of jumping in and saying "no"



    I will also re-iterate that you have demonstrated a lack of understanding re this topic and one question on here is not enough to redress that




    And you gave me a neg .... I pointed out that someone had told you the answer and you negged me ... seriously?
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by TenOfThem)
    I simply pointed out the bullet was a bad choice

    Your response was not polite ... and I pointed out that you already had the answer


    I think you need to read responses to the question, think about them, and absorb the maths advice instead of jumping in and saying "no"



    I will also re-iterate that you have demonstrated a lack of understanding re this topic and one question on here is not enough to redress that
    Hey I'm sorry for being rude to you - I was annoyed with steve's comment as it was rude. So I was quick to jump to conclusions when reading your comments. So I'm sorry for that.

    But I understand where I wen't wrong on that and have just finished 5 other completing the square questions, so I understand what I did wrong - Just made small mistake. But I apologize to you.

    And thanks for everyone who answered the question.
    Offline

    16
    ReputationRep:
    (Original post by Tweaky)

    But I understand where I wen't wrong on that and have just finished 5 other completing the square questions, so I understand what I did wrong - Just made small mistake.

    Fair enough


    The error you made is the single most common mis-understanding in this topic so do make sure that you understand why and that you are not just copying Raheem's example
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by TenOfThem)
    Fair enough


    The error you made is the single most common mis-understanding in this topic so do make sure that you understand why and that you are not just copying Raheem's example
    I use my-maths.co.uk so I'll do the other homework on there
 
 
 
  • 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
    Has a teacher ever helped you cheat?
    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

    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.