I. Circles

A. circle – a closed plane figure all of whose points are equidistant from a fixed point within,

 called the center; named by its center point. There are 360° in a circle.

     1. radius – (plural: radii) – a line segment from the center of the circle to a point on the circle

      2. chord – a line segment whose endpoints are on the circle

      3. diameter – a chord that passes through the center of the circle

     a. the longest chord of the circle

     b. cuts the circle into two congruent parts called semicircles

c. The length of a diameter is twice the length of a radius.

4. central angle – an angle whose vertex lies at the center of the circle

       a. arc – part of a circle

       1. The measure of an arc equals the measure of its corresponding angle.

      5. circumference – (C) the distance around a circle

     a. To find the approximate circumference of a circle use the following formula:

C = pd or C = 2pr

b. p (pi) is a Greek letter Mathematicians use that is approximately equal to 3.14 or .

     6. area of a circle – (A) the number of square units that cover the surface

a. To find the approximate area of a circle use the following formula: A = p r²

Circle

C = pd

C = 2pr

A = pr²

p = 3.14 or