Centuries: 6th century - 7th century - 8th century
Decades: 630s 640s 650s 660s 670s - 680s - 690s 700s 710s 720s 730s
Years: 684 685 686 687 688 - 689 - 690 691 692 693 694
Births
Deaths
... are also called the roots of the equation. The formula reads <math> x=\frac{-b \pm \sqrt {b^2-4ac}}{2a} </math> The term b2 – 4ac is called ...