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In a triangle the sum of the angles is 180

Posted March 19th, 2008 by Isoscel

In any triangle the sum the angles is 180.

In triangle ABC:
$ \angle ABC + \angle BCA + \angle CAB = 180. $

Proof
Let consider line DE by A parallel to BC
Then $ \angle DAC=\angle ACB $ and $ \angle EAB=\angle ABC $ but $ \angle DAC+\angle CAB +\angle EAB=180 $

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