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​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.

Types of Triangles

3x6x4 Triangle
6 + 3 > the third side 4
4+3 > the third side 6
6+4 > the third side 3
Interior angles = 180
60 + 60 +60  = 180 degrees
Measure exterior angle triangle
Interior and Exterior angles are supplementary
equilateral triangle
Theorem 2. The sum of the interior angles of a triangle equal 180 degrees.
​Theorem 3. The measure of an exterior angle 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.

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triangle with 50 30 110 degree angles
​Longest Side
​Shortest Side
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