areas of regular polygons 10-3. honor’s on a sheet of warm up paper: write the name of your...
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Areas of Regular Polygons
10-3
Honor’s
On a sheet of warm up paper:• Write the name of your podcast
group members (don’t write your own name)
• Rate each member from 1-10, 10 being “very helpful”
Area Formulas
Rhombus: .5(D1)(D2)
Trapezoid: .5(h)(b1+b2)
Kite: There ain’t one, use your common sense!
Definitions
• Center – the center of the circle circumscribed about the polygon
• radius – a segment drawn from the center of a polygon to a vertex
• apothem – a segment drawn from the center of a polygon that is perpendicular to a side
• central angle – an angle formed by two radii drawn to consecutive vertices
radi
us
center
apothem
Central angle
Theorem 11.6 Area of a Regular Polygon
• The area of a regular n-gon with side lengths (s) is half the product of the apothem (a) and the perimeter (P), so
A = ½ aP, or A = ½ a • ns.
NOTE: In a regular polygon, the length of each side is the same. If this length is (s), and there are (n) sides, then the perimeter P of the polygon is n • s, or P = ns
The number of congruent triangles formed will be the same as the number of sides of the polygon.
More . . .
• A central angle of a regular polygon is an angle whose vertex is the center and whose sides contain two consecutive vertices of the polygon. You can divide 360° by the number of sides to find the measure of each central angle of the polygon.
• 360/n = central angle
Ex: Finding the area of a regular polygon
• A regular pentagon with radius 1 unit. Find the area of the pentagon.
B
C
A
1
1D
Solution:
• you must find the apothem (or if the apothem was given, you must find the radius, etc)
• You need to find measure of central angle. ABC is 360°/5, or 72°.
1
DA C
B
Solution:
• Draw the apothem. It is an isosceles triangle so it bisects the angle.
• You now have a right triangle and can use trig ratios to find the missing sides
1
DA C
B
36°
Solution
1
DA C
B
You try…..
• Find the area of a regular polygon with 9 sides and a radius of 10
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