Angles in Triangle Add to 180^o

Based on the Parallel postulate, Euclid proves in Proposition I.32 - the basic fact of Euclidean geometry - namely, that the sum of the angles in a triangle is always equal to "two right angles". The proof is not difficult, although Sir Thomas L. Heath in his commentary gives an additional proof that is even simpler. The applet below illustrates a proof by "paper folding".

Let A denote the vertex with the largest (or one of the largest) angles. Then the feet D of the altitude AD is located on the base BC, between B and C. D is constructed via Axiom O4. Folds BC, EG and FH exist due to Axiom O2.

Now, why the construction works? AD BC and EF AD, we see that EF||BC and that AD is divided by EF into equal halves. Therefore, EF is a midline of DABC:

By construction, also, AE = DE and BE = DE. Therefore, the edges DE of the two foldings at EF and EG coincide. Similarly for DF and the foldings EF and FH. Thus the three angles at D, on one hand, add to two right angles and, on the other, are equal to the angles of DABC.

Paper Folding Geometry

• Angle Trisection by Paper Folding
• Angles in Triangle Add to 180^o
• Broken Chord Theorem by Paper Folding
• Dividing a Segment into Equal Parts by Paper Folding
• Egyptian Triangle By Paper Folding
• My Logo
• Paper Folding And Cutting Sangaku
• Parabola by Paper Folding
• Radius of a Circle by Paper Folding
• An Interesting Example of Angle Trisection by Paperfolding

Copyright © 1996-2009 Alexander Bogomolny