Circle : definitions, notes and formulas

What is it? A circle is a shape consisting of points — in the same plane — that are equidistant from a center point.

Circle definitionsParts of a circle:

Radius (r): Distance from any point on the circle to the Center (c)

Diameter (d): Length of any line segment that connects two points on the circle AND passes through the center

Diameter = 2 x Radius

The center is the midpoint of any diameter.

Area of the circle:   π  (radius)² = π r²

Circumference: the “perimeter” of the circle:    π (diameter)  =  π d

or  2 π (radius) = 2 π r

π is an irrational number approximately 3,1415

Example: Given a circle with radius 6 inches

area = π r² = π (6 inches)² = 36 π square inches
(approximately 113.1 sq. inches)

circumference = π d = π  (12 inches) = 12 π inches
(approximately 37.7 inches)


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