Centuries: 2nd century - 3rd century - 4th century
Decades: 200s 210s 220s 230s 240s - 250s - 260s 270s 280s 290s 300s
Years: 251 252 253 254 255 - 256 - 257 258 259 260 261
Births
Deaths
... b/(2a) from both sides, we get <math>x=\frac{-b}{2a}\pm\frac{\sqrt{b^2-4ac}}{2a}=\frac{-b\pm\sqrt{b^2-4ac}}{2a}.</math> ...