Question

1. prove that the total measure of the three angles of a triangle is 180 degrees?​

Answer 1

Answer:

So, the 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 (they are along the line l). So, their angle sum is 180°.