## Precalculus (6th Edition) Blitzer

RECALL: The standard form of the equation of a circle is given as $(x-h)^2+(y-k)^2 = r^2$ where $(h, k)$ is the center $r$ is the length of the radius (distance of each point on the circle from the center). The radius of a circle cannot be negative as it would mean that the distance of the points on the circle from the center is negative. This means that the graph of the given equation is made up of imaginary numbers.