Triangles Contain 180°

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

Proof the angles in a triangle add to 180

Explanation 1: 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, and
angles "b" are the same,

and you can easily see that "a" + "b" + "c" does a complete rotation from one side of the straight line to the other, or 180°

Explanation 2: From the rules of angles, when a line cuts two parallel lines we can show that the angles of the triangle a + b + c = the angle on a straight line = 180°