http://www.ck12.org Chapter 6. Polygons and Quadrilaterals
Make sure none of the triangles overlap.
- Make a table with the information below.
TABLE6.1:
Name of Polygon Number of Sides Number of (Column 3) Total Number of
Degrees
Quadrilateral 4 2 2 × 180 ◦ 360 ◦
Pentagon 5 3 3 × 180 ◦ 540 ◦
Hexagon 6 4 4 × 180 ◦ 720 ◦- Do you see a pattern? Notice that the total number of degrees goes up by 180◦. So, if the number sides isn, then
the number of triangles from one vertex isn−2. Therefore, the formula would be(n− 2 )× 180 ◦.
Polygon Sum Formula:For anyn−gon, the sum of the interior angles is(n− 2 )× 180 ◦.
Aregular polygonis a polygon where all sides are congruent and all interior angles are congruent.
Regular Polygon Formula:For anyequiangularn−gon, the measure of each angle is(n−^2 )×^180
◦
n.Example A
Find the sum of the interior angles of an octagon.
Use the Polygon Sum Formula and setn=8.
( 8 − 2 )× 180 ◦= 6 × 180 ◦= 1080 ◦
Example B
The sum of the interior angles of a polygon is 1980◦. How many sides does this polygon have?
Use the Polygon Sum Formula and solve forn.
(n− 2 )× 180 ◦= 1980 ◦
180 ◦n− 360 ◦= 1980 ◦
180 ◦n= 2340 ◦
n= 13 The polygon has 13 sides.Example C
How many degrees does each angle in an equiangular nonagon have?
First we need to find the sum of the interior angles in a nonagon, setn=9.
( 9 − 2 )× 180 ◦= 7 × 180 ◦= 1260 ◦
Second, because the nonagon is equiangular, every angle is equal. Dividing 1260◦by 9 we get each angle is 140◦.
Watch this video for help with the Examples above.