Theorem : If a side of a triangle is produced, then the exterior angle so formed is equal to the sum of the two interior opposite angles.

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Given      :     A ▲ABC whose side BC has been produced to D       forming exterior angle ∠4
To Prove :    ∠4 = ∠1 + ∠3
Proof       : We have, ∠1 + ∠2 + ∠3 = 180°
                                               ( Sum of the angles of a triangle)
                 Also,         ∠2 + ∠4 = 180°   
                                               ( Linear pair)
                 ∴               ∠2 + ∠4 = ∠1 + ∠2 + ∠3 
                 Hence,      ∠4 = ∠1 + ∠3.

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