Centuries: 6th century 7th century 8th century
Decades: 610s - 620s - 630s - 640s - 650s - 660s - 670s - 680s - 690s - 700s - 710s
Years: 660 661 662 663 664 665 666 667 668 669 670
Events:
... 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, ...