Question 1181777
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<pre>

Let  {{{alpha}}}, {{{beta}}}  and  {{{gamma}}} are three interior angles of the triangle.


Then the respective exterior angles are  {{{180^o - alpha}}},  {{{180^o - beta}}}  and  {{{180^o - gamma}}}.


The sum of exterior angles is then


    {{{180^o - alpha}}} + {{{180^o - beta}}} + {{{180^o - gamma}}} = {{{540^o -  (alpha+beta+gamma)}}} = 540° - 180° = 360°,


since the sum of interior angles of any triangle is 180°.
</pre>

At this point, the proof is completed.