1. Circles and Area

2. 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

3. 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

4. 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

5. 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 = cm 2 = cm 2

6. Work this problem Find the area of circle K. So, A = r 23
So, A = r 2
A = (3.14)(3 in.) 2
A = (3.14)(9 in.2)
A = in.2 = 28.3 in.2