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Tri Angles

Problem

Prove that exterior angle of a triangle is equal to the sum of two opposite interior angles.


Solution

Consider the following triangle.


Clearly a + b + c = 180o (angles in a triangle).

Also x + c = 180o (angles on a straight line).

Therefore a + b = x (Q.E.D.).

Problem ID: 243 (19 Oct 2005)     Difficulty: 2 Star

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