Property 3: Exterior Angle Theorem An exterior angle of a triangle is equal to the sum of the two opposite interior angles. The exterior angle theorem is amongst the most basic theorems of triangles in geometry. For more on this see Triangle external angle theorem. Exterior Angle Property. If the figure is regular all the exterior angles will be equal. A polygon is called a plane figure that is bounded by the finite number of line segments for forming a closed figure. The exterior angle theorem is Proposition 1.16 in Euclid's Elements, which states that the measure of an exterior angle of a triangle is greater than either of the measures of the remote interior angles. Exterior Angle Theorem. An exterior angle of a triangle is equal to the sum of the opposite interior angles. Similarly, this property holds true for exterior angles as well. Example: . Let us discuss this theorem in detail. Properties. Property 2: The sum of an interior angle and its adjacent exterior angle is 180°. Types of triangles. For a triangle: The exterior angle d equals the angles a plus b.; The exterior angle d is greater than angle a, or angle b. ; If the equivalent angle is taken at each vertex, the exterior angles always add to 360° In fact, this is true for any convex polygon, not just triangles. Exterior Angle Theorem. If the equivalent angle is taken at each vertex, the exterior angles always add to 360° In fact, this is true for any convex polygon, not just triangles. Triangles can be classified in 2 major ways: Classification according to internal angles Define an exterior angle of a triangle and identify all the exterior angles. Also, each interior angle of a triangle is more than zero degrees but less than 180 degrees. One of the basic theorems explaining the properties of a triangle is the exterior angle theorem. The side opposite to the largest angle of a triangle is the largest side. Before we begin the discussion, let us have look at what is a triangle. For more on this see Triangle external angle theorem. Learn the concepts of Class 7 Maths The Triangle and Its Properties with Videos and Stories. This is a fundamental result in absolute geometry because its proof does not depend upon the parallel postulate.. Prove that it is equal to the sum of the interior angles on the other two arms. Any exterior angle of the triangle is equal to the sum of its interior opposite angles. In this article, we will learn about: Triangle exterior angle theorem, exterior angles of a triangle, and, how to find the unknown exterior angle of a triangle. Same goes for exterior angles. Exterior Angle Property - Displaying top 8 worksheets found for this concept.. An exterior angle of a triangle is equal to the sum of the opposite interior angles. Apply this relationship to find unknown angles. This is called the exterior angle property of a triangle. Properties. 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