This lesson covers the equation of a circle, or (x - h)^2 + (y - k)^2 = r^2, where (h, k) is the center of the circle, and r is the radius. In the problems in this lesson, students are given the equation of a circle and are asked to find the center and the radius, then graph the circle. When graphing circles, start with the center, then use the radius to plot points above, below, to the left, and to the right of the center, then connect these points with a circle. Students are also given the center and the radius of a circle, and are asked to write the equation of the circle. Finally, students learn to use completing the square to write the equation of a circle in standard form, then find the center and radius. The equation of a circle is also called the circle formula or the standard form of a circle.