See: List of castles Castles in England
Angus
Argyll and Bute
Dumfries and Galloway
Dundee
Edinburgh
Fife
Highland
Moray
Perthshire
Scottish Borders
Stirling
... that will make it a perfect 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) t ...