Equation of a Circle

Learning Target: Write the equation of a circle and graph a circle from it's equation.

​With center of circle at origin of coordinate plane, Circle Equation is:
x² + y² = r² or

(x - h)² + (y - k)² = r²

Equation of a circle with center at (0,0)

What if the center isn't (0,0)?

We use the point (h,k) to talk about the center of the circle..

Equation of a circle

Writing Equations of Circles

We can write equations of circles from their graphs.

Locate the center of the circle. This usually marked with a dot

Writing Equations of Circles

The other piece of information we need is the radius.

You can calculate the radius by counting the number of units from the center to any point on the circle.

Writing Equations of Circles