What is a triangle? In addition to having three sides and three angles,there are four basic rules or theorems that a shape must satisfy in order to be classified as a triangle.

Theorem 1. This theorem states that when adding the side lengths of a triangle, the sum of two sides of a triangle must be greater than the length of the third side.

Theorem 2. The sum of the interior angles of a triangle equal 180 degrees.

Theorem 3. The measure of an exteriorangle is formed by extending one of the sides. Furthermore, the interior and exterior angles created, are supplementary.

Theorem 4.Proportionality of Triangles. The longest side of a triangle is opposite the largest angle, and the shortest side is opposite the smallest angle.