Picture proof that the area of a right triangle is $xy$
Let A be the area of the original triangle cut up in the obvious way as on the picture. Then the area of the rectangle is $A_r = 2 A = ab = xy + A$. Thus $xy = A$.
Let A be the area of the original triangle cut up in the obvious way as on the picture. Then the area of the rectangle is $A_r = 2 A = ab = xy + A$. Thus $xy = A$.