What’s the most elegant proof?
My favorite is this graphical one:
According to cut-the-knot:
Loomis (pp. 49-50) mentions that the
proof “was devised by Maurice Laisnez,
a high school boy, in the
Junior-Senior High School of South
Bend, Ind., and sent to me, May 16,
1939, by his class teacher, Wilson
The proof has been published by Rufus
Isaac in Mathematics Magazine, Vol. 48
(1975), p. 198.
I really like this one (image taken from cut-the-knot #4)
It’s so clear and easy …
Note: Just to make it clear, the side of the square is a+b