Prove that in each triangle the total angles are 180 degrees

This is a proof that the angles in a triangle equal 180°:

The top line (that touches the top of the triangle) is

running parallel to the base of the triangle.

So:

angles A are the same angles B are the same

And you can easily see that A + C + B does a complete rotation from one side of the straight line to the other, or 180°