Consider a piece of paper having shape of a triangle which we denote by triangle ABC.

Using paper folding alone, prove that the sum of the interior angles of the triangle ABC equals 180^o.

Bisect each of the angles to find the incenter.
Fold each of the corners in to the incenter.

The angles across from C is equal to C etc.

So 2A + 2B + 2C = 360
So A+B+C = 180

Posted by Jer on 2014-09-16 12:07:39