# Finding Center And Radius of Circle

• May 12th 2008, 01:55 PM
Hibijibi
Finding Center And Radius of Circle
x^2 + y^2 + 6x=0. How would I go about finding the radius and center? What's the basic method?

-Thanks!
• May 12th 2008, 02:13 PM
Moo
Hello,

Quote:

Originally Posted by Hibijibi
x^2 + y^2 + 6x=0. How would I go about finding the radius and center? What's the basic method?

-Thanks!

To complete the squares :D

I'll do it in a newly way to me... lol

$x^2+6x=x(x+6)=((x+3)-3)((x+3)+3)=(x+3)^2-3^2=(x+3)^2-9$
(I know you can do it in a shorter way... find it ! :D)

Hence the equation becomes :

$(x+3)^2+y^2-9=0 \Longleftrightarrow (x+3)^2+y^2=9$

We know that the equation of a circle, center $O(x_O,y_O)$ and radius R is :

$(x-x_O)^2+(y-y_O)^2=R^2$

Conclude (Sun)