Circles and Area

Definitions Circle – the locus (set) of all points in a plane equidistant from a given point called the center. A circle is usually named by the center point. D Circle Center Point

Definitions C Diameter – a chord that passes through the center point. CE is a diameter F D Radius – any segment with endpoints that are the center and a point on the circle. CF, EF, and DF are all radii. E

Some Important Facts! Diameter is twice the radius. D = 2R Radius is one-half the diameter R = ½ D Area of a circle is given by A = r2

Work this problem! The diameter of Circle X is 22 cm Remember, we need the radius which is ½ the diameter. Radius = ½ Diameter X Radius = ½ (22 cm) = 11 cm A =  r 2 A = (3.14)(11 cm) 2 A = (3.14)(121 cm 2) A = 379.94 cm 2 = 379.9 cm 2

Work this problem Find the area of circle K. So, A = r 2 3 So, A = r 2 A = (3.14)(3 in.) 2 A = (3.14)(9 in.2) A = 28.26 in.2 = 28.3 in.2