triangle, or simply a triangle. The lines that bound the triangle are called its sides, and the sum of their

lengths is its perimeter.

a. An angle that is formed within a triangle by any two sides is called an interior angle. An

angle that is formed outside a triangle by any side and the extension of another side is called an exterior

angle. Interior and exterior angles of a triangle are shown in Figure 3-9. Whenever the angles of a

triangle are mentioned, the interior angles are referenced unless otherwise specified. The interior angles

that are not adjacent to a given exterior angle are called opposite interior angles.

b. One side of a triangle is usually drawn as a horizontal line with the other two sides above it.

The bottom side is then called the base of the triangle, as shown in Figure 3-10, page 3-6. However, any

side of a triangle can be designated as its base, and it is not necessary to draw it at the bottom of the

figure. The angle opposite the base is called the vertex angle, and the vertex of this angle is called the

vertex of the triangle. The perpendicular distance from the vertex of a triangle to its base, or to an

extension of its base, is called the altitude of the triangle. The side of a triangle that is opposite a right

angle is called the hypotenuse.