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DEFINITION: closed plane figure formed by 3
or more line segments such that each
segment intersects exactly 2 other segments
only at endpoints
These figures are not polygons
These figures are polygons
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DEFINITION: each endpoint of a side of a
polygon
◦ Plural is vertices
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Convex: A polygon such that no line
containing a side of the polygon contains a
point in the interior of the polygon
Non-Convex (concave)- a polygon that is not
convex
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Equilateral- polygon with all of its sides
congruent
Equiangular- polygon with all of its interior
angles congruent
Just like we learned with triangles… but now
applies to all polygons!
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Regular- polygon that has all sides and
angles congruent
Irregular- two sides or interior angles are not
congruent
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Diagonal-segment that joins two
nonconsecutive vertices of a polygon
Polygon Names
3 sides
Triangle
4 sides
Quadrilateral
5 sides
Pentagon
6 sides
Hexagon
7 sides
Heptagon
8 sides
Octagon
9 sides
Nonagon
10 sides
Decagon
12 sides
n sides
Dodecagon
n-gon
8
The sum of all the exterior angles in a figure
is 360. So to find the measure of EACH
exterior angle, you divide 360 by the number
of sides.
360
 On your chart for n-gon please write n
under the measure of exterior angle of regular
polygon.
 Then fill in the rest of the columns.
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Each exterior and interior angle form
supplementary angles.
So take 180 – exterior angle to get each
interior angle.

Complete page 2 of your notes by yourself.
Then check your answers with the person
sitting next to you.
We can divide a polygon into triangles by
drawing the diagonals from 1 vertex.
Ex. Draw the diagonals from vertex A of
pentagon ABCDE.
You should have formed 3 triangles.
Draw the diagonals from vertex T in the
hexagon PQRSTU.
You should have formed 4 triangles.
Compare the # of sides in the polygon with the
# of s formed. There are 2 fewer s than
sides!!
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Therefore, the sum of all of the (interior)
angles of a n-gon is (n - 2)180
So fill in these blanks….
The sum of the angles of a pentagon is
_______  180 = __________
The sum of the angles of a hexagon is
________  180 = __________
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Continue working through your note sheet.
If you have questions raise your hand.