Question

prove that sum of the angles of a triangle is 180*

Answer 1

Draw line a through points A and B. Draw line b through point C and parallel to line a.


Since lines a and b are parallel, <)BAC = <)B'CA and <)ABC = <)BCA'.
It is obvious that <)B'CA + <)ACB + <)BCA' = 180 degrees.
Thus <)ABC + <)BCA + <)CAB = 180 degrees.