Circle equations

The default equation of a circle looks like this: (x−h)² + (y−k)² = r² where r is the radius of the circle, and the x and y binomials indicate where the center of the circle is.

• The sign inside of the equations will be the opposite from how the circle moves from the origin. (x-3)means that the center of the circle is located at x = 3, rather than x = -3.
• This is true for both the x and y portions of the equation, rather than just the x term

Circle questions on the ACT math section will normally provide either a graph or an equation. Whichever one is provided must be used to find the one that isn’t provided.