 # Special Triangles

Math
Summary

## Classifying Triangles

Summary Classifying Triangles

### Parts of Triangles

Every triangle has three sides and three angles. In the following lessons we'll refer to certain sides as opposite sides, and certain angles as included angles. It's important to understand these definitions as early as possible. A side is opposite an angle, or a vertex, if neither of the endpoints of that side are at the vertex of the specified angle. An angle is included between two sides if the common endpoint of the sides is the vertex of the angle. Below these concepts are pictured. Figure %: A generic triangle, triangle ABC In triangle ABC above, side a is opposite angle A, side b is opposite angle B, and side c is opposite angle C. Angle A is included in sides b and c, angle B is included in sides a and c, and angle C is included in sides a and b.

When triangles are classified according to the lengths of their sides, they fit into one of three categories: scalene, isosceles, or equilateral. If none of the sides of a triangle are equal (of equal length), the triangle is scalene. If two or more of the triangles sides are equal, the triangle is isosceles. If all three of the sides of a triangle are equal, it is equilateral. All equilateral triangles are also isosceles, by definition.