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.

Let AB be the longest side.

There is a point on AB where folding A, B and C all to this point would line these angles up and show the sum is 180 degrees.

To find this point make small creases at the midpoints of AC and BC  (found by folding A to C and B to C) then fold C to AB using these midpoints.  [technically you only need one midpoint.]

Posted by Jer on 2014-09-16 11:52:06