He also does a few simple images for Wikipedia. Some of the older ones are collected at http://www.zundark.btinternet.co.uk/pdimages.htm .
... square trinomial of the form x2 + 2xy + y2. Since "2xy" in this case is (b/a)x, we must have y = b/(2a), so we add the square of b/(2a) to both sides, ...