A circle is the set of all points equidistant from a given point. The point from which all the points on a circle are equidistant is called the center, and the distance from that point to the circle is called the radius.

The circle above has its center at point C and a radius of length r. All circles also have a diameter. The diameter of a circle is a segment that contains the center and whose endpoints are both on the circle. The length of the diameter is twice that of the radius.

It is almost certain that you will encounter an SAT question or two that will in some way test your ability to find the circumference of a circle. The formula to find the circumference of a circle is where r stands for the length of the radius. Because two times the radius is also equal to a circle’s diameter, the formula for the circumference of a circle can also be written as

This is one of the equations found in the reference section of every math SAT section. You should memorize it, but if you do forget it, you’ve got back up.

The area of a circle is the square of the radius multiplied by Again, this formula can be found in the reference bar at the top of each SAT math section, but you should memorize it for the sake of efficiency, and because we told you to.

An arc of a circle consists of two points on the circle and of the points on the circle that lie between those two points. It’s like a line segment that has been wrapped partway around a circle. An arc is measured not by its length (although it can be, of course) but most often by the measure of the angle whose vertex is the center of the circle and whose rays intercept the endpoints of the arc. Hence, an arc can be anywhere from 0 to 360 degrees.

A chord is a line segment whose endpoints are on a circle. A diameter is a special chord that includes the center, but note that a chord does not have to include the center.

A chord and two radii, each extending from one endpoint of the chord to the center of the circle, form an isosceles triangle.