This is a question about elementary geometry, so I think it belongs on this site.
Let $d$ be the length of a diagonal in a rectangle, and let $m$ be half the perimeter.
Then a formula for the area of the rectangle is given by $$A=\frac 1 2 (m^2-d^2).$$
I can easily prove the formula using algebraic manipulations, but I feel it should be possible to see directly from the diagram that the size of the rectangle is half the difference between the sizes of those squares.
So my question is:
What is a simple geometric (and visual) proof of the formula?