Geometry
SHAPES AND FIGURES OF PLANE GEOMETRY
Circles
Figure 13 Circle
A circle is a plane curve which is equidistant from the
center, as shown in Figure 13.
The length of the
perimeter of a circle is called the circumference. The
radius (r) of a circle is a line segment that joins the
center of a circle with any point on its circumference.
The diameter (D) of a circle is a line segment connecting
two points of the circle through the center. The area of
a circle is calculated using the following formula:
A = pr2
(3-9)
The circumference of a circle is calculated using the
following formula:
C = 2pr
(3-10)
or
C = pD
Pi (p) is a theoretical number, approximately 22/7 or 3.141592654, representing the ratio of the
circumference to the diameter of a circle. The scientific calculator makes this easy by designating
a key for determining p.
The area of a circle is always expressed in square units, and the perimeter of a circle is always
expressed in the original units.
Example:
Calculate the area and circumference of a circle with a 3" radius. Be sure to include units
in your answer.
Solution:
A = pr2
C = 2pr
A = p(3)(3)
C = (2)p(3)
A = p(9)
C = p(6)
A = 28.3 square inches
C = 18.9 inches
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MA-03
