Explained in the [url=https://youtu.be/Rpjab--XQ0U]Numberphile[/url] video featuring 2006 Fields Medallist Andrei Okounkov.

You can stop the animation and move the three points defining the two adjacent squares.[br][br]Of course, Pythagoras theorem gives you the size of the tiling square, whose area is composed of the union of the area of both squares!