Let us consider a triangle ∆ ABC

 

Let ∠ 1 be the exterior angle

13.jpg

 

Since BAD is a line

  ∠ A + ∠1 = 180°      (Linear Pair)

  ∠ 1 = 180° − ∠ A

 

So, in a polygon

  Interior angle + Exterior angle = 180°

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Davneet Singh

Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 14 years. He provides courses for Maths, Science and Computer Science at Teachoo