Question
Mon October 01, 2012 By: Prashanth Balasubramaniam

Prove that sum of angle of triangle are 180

Expert Reply
Mon October 01, 2012

Consider a triangle ABC.

To prove, angle a + angle b + angle c = 180 degrees

Through vertex A, draw a line l parallel to BC.

 

 

Since alternate interior angles are equal so;

Angle b = angle x                     … (1)

Angle c = angle y                      … (2)

 

Since l is a straight line so, angle x + angle b + angle y = 180 degrees

Therefore, angle b + angle a + angle c = 180 degrees    [Using (1) and (2)]

That is angle a + angle b + angle c = 180 degrees

Hence, proved.

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