When a triangle is labelled a, b, c, how can you prove it has a sum of 180 degrees?

You can prove that a triangle has a sum of 180 degrees when labelled a, b, c, by drawing one of the bottom exterior angles with a line going through it and labelling these two angles x and y, on a straight line, now state that the angle at the top of the triangle is alternate to x, making the other bottom angle corresponding to y, and as c + x + y = 180 degrees, being on a straight line, a + b + c = 180 degrees