Answer:
We consider a triangle ABC as attached in the answer.
We draw a line parallel to AB that passes through point C.
Angles C and C' are vertical angles, therefore ∠C = ∠C'.
Angles B and B' are corresponding angles, therefore ∠B = ∠B'.
Angles A and A' are corresponding angles, therefore ∠A = ∠A'.
it can be seen that angle sum ∠A + ∠B + ∠C is equal to the angle sum ∠A' + ∠B' + ∠C'.
The three angles A', B', and C' form together a straight angle so, their angle sum is 180°. But then the angle sum ∠A + ∠B + ∠C must also be 180°.
This proofs that sum of angles of a triangle is equal to 180.
