Decades: 1210s 1220s 1230s 1240s 1250s - 1260s - 1270s 1280s 1290s 1300s 1310s
Years: 1256 1257 1258 1259 1260 - 1261 - 1262 1263 1264 1265 1266
Births
Deaths
... 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 add the square of ...