Below is a topology loosely based on the Library of Congress Classification System (http://lcweb.loc.gov/catdir/cpso/lcco/lcco)
... expression to the left of "=", 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 ...