Rule
This means that all values for that fulfill the equation are on the circle boundary.
To find the radius of the circle you can use this formula:
Example 1
You are working with a circle with its center at . The point is on the circle boundary. What is the radius of this circle?
Here you have and the point on the circle is . The radius of the circle is therefore
Example 2
Show that is a circle
To show this, you have to use the method to complete the square. You add and subtract for both the -terms and the -terms. You then get the following:
You can see from this that is a circle with its center at and a radius .