Theorem: The exterior angle at a vertex of a triangle equals the sum of the interior angles at the other two vertices of the triangle.
Prerequisites:
Angle sum property of a triangle (proof)
Angle on a straight line (proof)
Proof:
In the given triangle, \angle CBD is an exterior angle at the vertex B. Since AD is a straight line,
\therefore\quad\;\;\angle CBD + \angle ABC = 180^o
\Rightarrow\quad\;\;\angle CBD = 180^o - \beta
Also, by the angle sum property of a triangle,
\qquad\quad\angle A + \angle ABC + \angle C = 180^o
\begin{align}\Rightarrow\quad\;\;\angle A + \angle C &= 180^o - \beta\\
&= \angle CBD\end{align}
Hence the result.
Recommended:
Angle sum property of polygon
Isosceles Triangle Theorem
Interior angles property
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