How to prove the sum of angles in a Triangle is 180°

We Know sum of angles in a tri angle is 180°  but how can we prove that Consider triangle ABC

Draw a line which is parallel to “AB” and passing through “C”

Their is new angles  ∠ACD and ∠BCE are formed

So from figure we understand that

            ∠ACD = ∠A    Alternate intern angles
            ∠BCE = ∠B     Alternate intern angles
 So
      Sum of angles of
                       triangle = ∠A+∠B+∠ACB
                                      = ∠ACD+∠BCE+∠ACB
                                      = 180° because
                                             DE is a straight line