POLYGONS Poly = “MANY” Gon = “ANGLES” OR “SIDES” 7 th Grade Math

POLYGONS! 1. Three (3) or more line segments 2. CLOSED figure 3. Line segments DO NOT cross or intersect

Polygon Examples: Do these meet the above 3 criteria?

Is this shape a polygon? Why or why not?

This shape is NOT “closed.”

Why can’t a polygon have only 2 sides?

With only 2 sides, the shape could not be “closed.”

Is this shape a polygon? Does it meet the 3 criteria?

YES, this IS a polygon.

This shape is two-dimensional, “closed”, has 12 sides, and lines do not intersect.

However, this is a special type of polygon called “concave.”

It’s called “concave” because some of the sides are “caved in.”

For our purposes, we will be discussing only convex polygons, not concave polygons.

REGULAR Polygon… 1. ALL sides congruent 2. ALL angles congruent Example: Square and Equilateral Triangle What would it mean if I said, “I have a regular pentagon”?

Let’s learn the names for the first eight polygons.

Polygons! Polygon Name PictureDefinitionPrefix & Memory Device

A one-sided polygon is called…?

Trick question!!! There is no such thing as a one-sided polygon. Remember, polygons must have at least 3 sides.

3-sided polygon… TRIANGLE.

You can remember the prefix “tri” by thinking of a tricycle. A tricycle has 3 wheels.

4-sided polygon… QUADRILATERAL.

You can remember the prefix “quad” by thinking “times four.” Quadruple means x 4

5-sided polygon… PENTAGON

You can remember this name by thinking about the building in Washington, D.C.

6-sided polygon… HEXAGON

You can remember that a hexagon has six sides because the words hexagon and six both have the letter “x.”

The hexagon is the polygon of choice for bees.

7-sided polygon… HEPTAGON.

You can remember the prefix “hept” by thinking of the heptathalon in the Olympics. heptathlon—a two-day event in which athletes compete SEVEN DIFFERENT EVENTS meter hurdles 2.high jump 3.shot put meter dash 5.long jump 6.Javelin meter race on the second day.

Joyner-Kersee, Jackie Jackie Joyner-Kersee, an American track-and-field athlete, who won the heptathlon event (an all- around event) at the Olympic Games in 1988 and She is considered one of the greatest female athletes. Joyner-Kersee, Jackie Jackie Joyner-Kersee, an American track-and-field athlete, who won the heptathlon event (an all- around event) at the Olympic Games in 1988 and She is considered one of the greatest female athletes. track-and-fieldOlympic Games track-and-fieldOlympic Games

8-sided polygon… OCTAGON

You can remember the prefix “oct” by thinking of an octopus (that has 8 legs).

9-sided polygon… NONAGON

You can remember that a nonagon has nine sides because the words nonagon and nine both have two “n s.”

10-sided polygon… DECAGON

You can remember the prefix “dec” by thinking about a decade, which has 10 years TEN YEARS

Sum of Interior Angles Discuss the angle measures for: Can you create a formula that would work? Shape# of sides How many triangles in this shape from one vertex? Degrees in a triangle?Degrees in Shape? Triangle31180 Quadrilaterial Pentagon Hexagon Heptagon Octagon Nonagon Decagon

Formula to calculate the total degrees in a polygon: n = number of sides (n-2)180

Scholastic “Sum of Interior Angles” Activity

Practice: GlencoePage 549 #1-5#7-16