Question

sum of angle of a triangle is 180​

Answer 1

Yes it is correct that sum of angle is 180•

Answer 2

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°.