You are Here: Home >< Maths

# Co-ordinate geometry watch

1. 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
2. (-6/-2, 4/-2) = (3,-2)
3. why are you divinding it by -2

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

4. (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.
5. Yeh, but how did you get (x - 3)^2. Why not (x - 2)^2

Im confused on such a basic question !!!!
6. (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.
7. Man, still confused.

Can you show me a step by step method of how you would do it?
8. (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)
9. Thanks Man,

All cleared up now!
10. (Original post by Bengaltiger)
Thanks Man,

All cleared up now!
Good.

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: June 18, 2005
The home of Results and Clearing

### 1,355

people online now

### 1,567,000

students helped last year
Today on TSR

### University open days

1. Sheffield Hallam University
Tue, 21 Aug '18
2. Bournemouth University
Wed, 22 Aug '18
3. University of Buckingham
Thu, 23 Aug '18
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams