Question

prove that the sum of a triangle is equal to 180° please answer it fast . thank you!!!!

Answer 1

hey friend!

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'.

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°. But then the angle sum ∠A + ∠B + ∠C must also be 180°.

hope it helps ✌

Answer 2

this the correct one