Question

prove that the sum of the angles of the triangles is 180 degree
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Answer 1

Let a line say DE be produced parallel to triangle ABC

1. Angle 4 + Angle 5 + angle 3 = 180 degree ( linear pair)
2. to prove: angle1 + angle 2 + angle 3 = 180

degr

3. angle 1 = angle 4 ( since AB is parallel to DE and AC is a line of transversal

4. angle 2 = angle 5 ( since AB is Parallel to DE and CB is a line of transverse )

Putting equations 3,4 in 2 , we get

angle 1+2+3 = 180 degree