This last lesson in this unit will demonstrate a few important properties of circles. When you first start learning about circles, you learn what **radius** and **diameter** mean. As a refresher, the *diameter* is the measurement across the circle passing through the center (shown below), while the *radius* is half that distance. Furthermore, a **chord** of a circle is a straight line segment whose endpoints both lie on the circle’s **circumference** (distance around the circle).

The first thing that you’ll watch is how a line segment drawn from the circle of a circle relates to a **chord** drawn between two points along the circle’s circumference. You’ll see that in a perfect circle, the line from the center to the chord is always at 90° to the chord, so they have slopes that are **negative reciprocals**.

Furthermore, our next video will show that you **cannot** determine the center of a circle from just two points on the circumference. Because the center can lie anywhere along the right bisector of a chord produced between two points, a third point is needed to verify.