Circumference and Area: Circles

Formulas C = πd or 2πr A = πr2

Definitions: Circle- the set of all points in a plane that are the same distance from a given point, called the center.

The distance from the center to any point on the circle is called the radius.

The length of the radius is equal to ½ the length of the diameter.

The distance across the circle through its center is the diameter.

The circumference is the distance around the circle.

pi ( ): A number, 3.141592..., equal to (the circumference) / (the diameter) of any circle.

For any circle with a diameter, d the circumference, C is found by using this formula

Remember that, for any circle, the diameter is twice the radius or d = 2r. If we are told the radius instead of the diameter we use the formula:

The circumference of a circle is the product of pi and the diameter (or twice the radius).

The area of a circle is the product of pi and the square of the radius.

Arc- a curved line that is part of the circumference of a circle.

Chord- a line segment within a circle that touches 2 points on the circle Diameter is a special chord that goes through the center.