ΔABC is a right triangle, so by the Pythagorean theorem, (1) AB˛+BC˛ = AC˛ ΔABE is a right triangle, so by the Pythagorean theorem, (2) AB˛+BE˛ = AE˛. ΔDBC is a right triangle, so by the Pythagorean theorem, (3) DB˛+BC˛ = CD˛. ΔDBE is a right triangle, so by the Pythagorean theorem, (4) DB˛+BE˛ = DE˛. Add BE˛+DB˛ to both sides of equation (1) AB˛+BC˛+BE˛+DB˛ = AC˛+BE˛+DB˛ Rearrange the terms: AB˛+BE˛+DB˛+BC˛ = AC˛+DB˛+BE˛ To make things easier to see, let's put parentheses around the first two terms on the left, the last two terms on the left, and the last two terms on the right: (5) (AB˛+BE˛)+(DB˛+BC˛) = AC˛+(DB˛+BE˛) Using (2) we replace (AB˛+BE˛) in (5) by AE˛ Using (3) we replace (DB˛+BC˛) in (5) by CD˛ Using (4) we replace (DB˛+BE˛) in (5) by DE˛ And we end up with what we were to prove: AE˛+CD˛ = AC˛+DE˛ Edwin