# Circle Geometry

1. May 24, 2008

### thomas49th

1. The problem statement, all variables and given/known data

The circle C, with the centre at the point A, has equation x² + y² - 10x + 9 = 0

Find:

a) the co-ordinates of A,

2. Relevant equations

(x-a)² + (y-b)² = r²

3. The attempt at a solution

... not sure what to do really. any suggestions

thanks :)

2. May 24, 2008

### Tedjn

Once you do one of these, you'll know how to do them all. Try completing the square for both variables.

3. May 24, 2008

### thomas49th

(x²-5) -16
(y²)+9=0

so does this mean (5,0) is the co-ordinate.
is the radius sqrt(16-9) = sqrt(7)

???

thanks

4. May 24, 2008

### Tedjn

You have a x2 and a y2. How would you get it into the form (x-a)2 and (y-b)2?

5. May 25, 2008

### thomas49th

sorry im not sure :( did i complete the square correctly?

6. May 25, 2008

### Defennder

It's sort of only a bit correct. The equation of a circle is $(x-x_0)^2 + (y-y_0)^2 = r^2$, where $x_0$ and $y_0$ are the coordinates of the centre and r is the radius.

Complete the square for x^2 - 10x + 9 first, then substitute the resulting expression back into the question, and move the constant to the RHS. You'll get the equation of circle.

7. May 25, 2008

Thanks!!! :)