1.6. Classifying Polygons http://www.ck12.org
You might have also learned that the sum of the interior angles in a triangle is 180◦. Later we will prove this, but for
now you can use this fact to find missing angles.
Example 1:Which of the figures below are not triangles?
Solution:Bis not a triangle because it hasone curved side.Dis not a closed shape, so it is not a triangle either.
Example 2:How many triangles are in the diagram below?
Solution:Start by counting the smallest triangles, 16. Now count the triangles that are formed by four of the smaller
triangles.
There are a total of seven triangles of this size, including the inverted one in the center of the diagram.Next, count
the triangles that are formed by nine of the smaller triangles. There are three of this size. And finally, there is one
triangle formed by the 16 smaller triangles. Adding these numbers together, we get 16+ 7 + 3 + 1 =27.
Classifying by Angles
Angles can be classified by their angles;acute, obtuse or right. In any triangle, two of the angles will always be acute.
The third angle can be acute, obtuse, or right.