"What is the area? You should be able to solve this"

[Albert Chan]

Looking back my first post, I had proved opposite quadrilaterals sum to half of big square, without realizing it.

With assumed square side = 4, I had this: right top + left bottom = 8

Only one more step, saying area of square = 4 * 4 = 16 = 2 * 8, and the prove is done.

Oh, so close ...