The pages linked in the right-hand column contain lists of areas that are of the same order of magnitude (power of ten). Rows in the table represent increasing powers of a hundred (two orders of magnitude).
... does not depend on x) to the 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, ...