# Finding the center and radius of circles given an equation

1. Dec 9, 2008

### Jim4592

1. The problem statement, all variables and given/known data
Find the center and the radius of the circle with the given equation.

2. Relevant equations

x2+y2+4y-117=0

3. The attempt at a solution

I first got it in standard form by completing the square:

x2+(y+2)2=121

but i don't know how to get the center and radius of it? I'm pretty sure its given in that form of the equation, right? but i'm not sure which numbers are the ordered pair.

2. Dec 9, 2008

### symbolipoint

Your basic standard form for a circle equation is (x-h)2+(y-k)2=r2. That form lets you directly read how the center has been translated and also shows the radius.

3. Dec 9, 2008

### Georgepowell

The general equation for a circle is:

(x + a)² + (y + b)² = r²

where (-a,-b) is the centre of the circle and r is the radius.

That should complete your problem, as you have already put it mostly into the right form

4. Dec 9, 2008

### Jim4592

so the center of my circle is (0,-2) and would that make the radius 11, the square root of 121??

5. Dec 9, 2008

### tiny-tim

(hmm … everyone else seems to have gone out, so …)

Yup!