Learn the concepts of Class 7 Maths The Triangle and Its Properties with Videos and Stories. Any exterior angle of the triangle is equal to the sum of its interior opposite angles. Apply this relationship to find unknown angles. This is a fundamental result in absolute geometry because its proof does not depend upon the parallel postulate.. Property 3: Exterior Angle Theorem An exterior angle of a triangle is equal to the sum of the two opposite interior angles. Prove that it is equal to the sum of the interior angles on the other two arms. Exterior Angle Theorem. ∴ Exterior angle is equal to sum of interior opposite angles Find exterior angle ∠ 1 Here, ∠1 = ∠B + ∠C ( Exterior angle property) ∠1 = 45° + 60° ∠1 = 105° Find exterior angle In ∆ABC, ∠BCD = ∠CAB + ∠ABC (Exterior angle property) ∠BCD = 85° + 25° ∠BCD = 110° Find exterior angle In ∆PQR, For more on this see Triangle external angle theorem. Types of triangles. For more on this see Triangle external angle theorem. Properties. Let us discuss this theorem in detail. ; 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. 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. The side opposite to the largest angle of a triangle is the largest side. 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. Same goes for exterior angles. Triangles can be classified in 2 major ways: Classification according to internal angles One of the basic theorems explaining the properties of a triangle is the exterior angle theorem. A polygon is called a plane figure that is bounded by the finite number of line segments for forming a closed figure. 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. 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. Before we begin the discussion, let us have look at what is a triangle. Exterior Angle Property. Also, each interior angle of a triangle is more than zero degrees but less than 180 degrees. Define an exterior angle of a triangle and identify all the exterior angles. In any geometrical figure, whether regular or irregular, and with any number of sides, the sum of the exterior angles will always be 360 degrees. Properties. An exterior angle of a triangle is equal to the sum of the opposite interior angles. 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