The document discusses the standard form of the equation of a circle. It can be written as x2 + y2 = r2 if the center is at the origin (0,0), or (x - h)2 + (y - k)2 = r2 if the center is at a point (h,k). Several examples are provided of writing the equation of a circle given its center and radius. The document also discusses determining the radius if the circle is tangent to the x-axis, y-axis, or a line.