This proof is by weininjieda from Yingkou, China who plans to become a teacher of mathematics, Chinese and history. It was included as algebraic proof #50 in E. S. Loomis' collection, for which he refers to an earlier publication by J. Versluys (1914), where the proof is credited to Cecil Hawkins (1909) of England.
Let CE = BC = a, CD = AC = b, F is the intersection of DE and AB.
ΔCED = ΔABC, hence DE = AB = c. Since, AC BD and BE AD,ED AB, as the third altitude in ΔABD. Now from
Area(ΔABD) = Area(ΔABE) + Area(ΔACD) + Area(ΔBCE)
we obtain
c(c + EF) = EF·c + b² + a²,
which implies the Pythagorean identity.