So hmm check the picture below, that’s about the circle and the endpoints, but notice, the endpoints make up a segment, namely the diameter of the circle, well…. let’s see how long that is, because, the radius is half the diameter

now.. hmmm notice, the midpoint of the diameter, is the center of the circle, let’s check that one out

now.. that we know what the center is, and what the radius is, well