11.6 Areas of Regular Polygons
Goal • Find areas of regular polygons inscribed in circles.
VOCABULARY
Center of a polygon - The center of its circumscribed circle.
Radius of a polygon - The radius of its circumscribed circle.
Apothem of a polygon - The distance from the center to any side of the polygon.
Central angle of a regular polygon - An angle formed by two radii drawn to
consecutive vertices of the polygon.
Example: In the diagram, ABCDEF is a Area  Area( ) # of s regular hexagon inscribed in G. Find
each angle measure.
a. mEGF
b. mEGH
c. mHEG
THEOREM: AREA OF A REGULAR POLYGON
The area of a regular n-gon with side length s is half the product of the apothem a, side length s (one triangle)
multiplied by n (the number of triangles).
A = 1 _______ · _______.
2
A  Area( ) # of s
Example: Coaster A wooden coaster is a regular octagon with 3 centimeter
sides and a radius of about 3.92 centimeters. What is the area of the coaster?
Construct point S on segment PQ and use triangle PRQ to find RS
A  Area( ) # of s
Example: The radius of the regular pentagon is about 6.8 inches.
Find the area to the nearest square inch.
Example: A regular nonagon (9 sides) is inscribed in a circle with radius 5 units. Find the perimeter P and
area A of the nonagon.
A  Area( ) # of s
Example: Find the perimeter and area of the equilateral triangle inscribed in F.
FINDING LENGTHS IN A REGULAR N-GON
To find the area of a regular n-gon with radius r, you may need to first find the apothem a or the side length s.
You can use
when you know n and
Pythagorean Theorem:
1 I
F
G
H2 SJK a
2
2
 r2
Two measures: r and a, or r and s
Special Right Triangles
Any one measure: r or a or s And the value of n is 3, 4, or 6 (that is a
regular triangle, square, or regular hexagon)
(making 30-60-90s or 45-45-90s)
Trigonometry
SOH-CAH-TOA
Any one measure: r or a or s