Geometry Lesson Notes 1.6 Date ______
Objective: Identify, classify, and name polygons. Find the perimeter of polygons.
‘polygon’ comes from Greek word meaning ‘many-angled’
Polygon: a closed figure formed by three or more coplanar line segments called sides.
Each side intersects two other sides but only at their endpoints.
Notation: a polygon is named by the letter of its vertices, written in consecutive order.
Practice: Identify and name the polygons.
Convex: a polygon is convex if none of its sides can be extended into the interior of the polygon.
a polygon is convex if each segment connecting non-consecutive vertices lies is in the interior of the polygon.
a polygon is convex if it does not ‘cave in’.
Concave: a polygon is concave if some of its sides can be extended into the interior of the polygon.
a polygon is concave if a segment connecting non-consecutive vertices lies is in the exterior of the polygon.
a polygon is concave if it ‘caves in’.
Classifying polygons: polygons are classified by the number of sides.
Number of Sides Polygon
3 triangle
4 quadrilateral
5 pentagon
6 hexagon
7 heptagon
8 octagon
9 nonagon
10 decagon
12 dodecagon
n n-gon
Regular polygon: a convex polygon with all sides congruent and all angles congruent.
Example 1 (p 46): Identify Polygons
Name each polygon by its number of sides. Then classify it as convex or concave,
and regular or irregular.
What is missing from these drawings if we want to be able to classify the polygons?
Perimeter: the sum of the lengths of the sides of a polygon.
P = a + b + c P = s + s + s +s P = l + w + l + w
P = 4s P = 2l + 2w
Example 2 (p 47): Find Perimeter (Word Problem)
A masonry company is providing a quote for a job. The company will have to lay three layers of decorative brick along the foundation for a new vacation house given the dimensions below.
a. Find the perimeter of the foundation and determine how many bricks the company will need to complete the job. Assume that one brick is 8 inches long.
b. The customer realizes he cannot afford such a large structure and decides to cut all of the dimensions in half. How will this affect the perimeter of the house and the number of bricks the masonry company needs?
Example 3 (p 47): Perimeter on the Coordinate Plane
Find the perimeter of pentagon ABCDE with A(0, 4), B(4, 0), C(3, −4), D(−3, −4), and
E(−3, 1).
Example 4 (p 48): Use Perimeter to Find Sides
The width of a rectangle is 5 less than twice its length. The perimeter is80 cm. Find
the length of each side.
HW: A12 pp 49-50 #12-18 all, 19-35 odd, 38, 40-44
Prepare for Quiz 1.6
fms-Geometry Lesson Notes 1.6 Page 4 of 4