Triangle: Sum of Remote Interior Angles & Exterior Angle

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An exterior angle of a triangle is the angle formed by extending one of the sides of the triangle past a vertex (the point at which two sides meet). An exterior angle is always supplementary to the interior angle with which it shares a vertex and equal in measure to the sum of the measures of the remote interior angles. Take a look at the figure below, in which δ, the exterior angle, is supplementary to interior angle β: Can you discover this rule? You can move the labels α, β, and γ with your mouse to read.