Turn on thread page Beta
    • Thread Starter
    Offline

    0
    ReputationRep:
    This question should be easy......i just forgot how to do it!!

    what is the co-ordinates of the centre of this circle.

    x² + y² - 6x + 4y -12 = 0
    Offline

    1
    ReputationRep:
    (-6/-2, 4/-2) = (3,-2)
    • Thread Starter
    Offline

    0
    ReputationRep:
    why are you divinding it by -2

    And markscheme says attempt to complete the square. How would you do that in this case?

    thanks for quick reply
    Offline

    0
    ReputationRep:
    (Original post by Bengaltiger)
    This question should be easy......i just forgot how to do it!!

    what is the co-ordinates of the centre of this circle.

    x² + y² - 6x + 4y -12 = 0
    Completing the Square:

    (x - 3)² - 9 + (y + 2)² - 4 -12 = 0

    So: (x - 3)² + (y + 2)² = 25

    So: C-ords of centre: (3,- 2) and radius 5.
    • Thread Starter
    Offline

    0
    ReputationRep:
    Yeh, but how did you get (x - 3)^2. Why not (x - 2)^2

    Im confused on such a basic question !!!!
    Offline

    0
    ReputationRep:
    (Original post by Bengaltiger)
    Yeh, but how did you get (x - 3)^2. Why not (x - 2)^2

    Im confused on such a basic question !!!!
    divide x and y values (ie. -6x and +4y) by 2 put into brackets. Then take of square of each number you put in brackets.
    • Thread Starter
    Offline

    0
    ReputationRep:
    Man, still confused.

    Can you show me a step by step method of how you would do it?
    Offline

    0
    ReputationRep:
    (Original post by Bengaltiger)
    Man, still confused.

    Can you show me a step by step method of how you would do it?
    Ok, its broken down below:

    x² - 6x this goes to (x - 3)² - 9. That is gotten by halving -6x to give -3x. The x is ignored for the time being and the three is put in brackets with an x. To get (x-3)² (you then square the brackets). Now if this was squared out you would get x² - 6x + 9. There is an extra +9 so you subtract 9. ie the number (3) squared. This gives (x - 3)² -9

    The same is applied to the y's to give (y + 2)² - 4

    from the equation you had this gives.

    (x - 3)² - 9 (y + 2)² - 4 -12 = 0

    Thus: (x - 3)² + (y + 2)² = 25 when rearranged.

    So centre is found at (3,-2)
    • Thread Starter
    Offline

    0
    ReputationRep:
    Thanks Man,

    All cleared up now!
    Offline

    0
    ReputationRep:
    (Original post by Bengaltiger)
    Thanks Man,

    All cleared up now!
    Good.
 
 
 
Turn on thread page Beta
Updated: June 18, 2005
The home of Results and Clearing

1,355

people online now

1,567,000

students helped last year

University open days

  1. Sheffield Hallam University
    City Campus Undergraduate
    Tue, 21 Aug '18
  2. Bournemouth University
    Clearing Open Day Undergraduate
    Wed, 22 Aug '18
  3. University of Buckingham
    Postgraduate Open Evening Postgraduate
    Thu, 23 Aug '18
Poll
A-level students - how do you feel about your results?
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.