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:

... 2a is zero and it is impossible to divide by it.) The symbol <math>\pm \sqrt {b^2-4ac}</math> in the formula should be understood as "either of thos ...