About the Angles of a Right TriangleTo understand the Chinese proof one needs to know that the sum of the three angles of a triangle in the Euclidean plane is two right angles, that is, 180 degrees. Applying to a right triangle, as one of the angles is a right angle, the sum of the other two angles must be one right angle, or 90 degrees. Revised by Ralph Abraham, 21 Mar 2001 |