A.5 Why does it work?

Angles

Prove that the interior angles of a triangle sum to 180°.

Alex's "Corner Tearing" Method
I ripped the corners off the triangle.
Then, I moved the pieces and saw that the angles create a straight line.
I already know a line is 180°, so the three angles of a triangle must add up to 180°.
m∠1 + m∠2 + m∠3 = 180°