Decades: 1170s 1180s 1190s 1200s 1210s - 1220s - 1230s 1240s 1250s 1260s 1270s
Years: 1221 1222 1223 1224 1225 - 1226 - 1227 1228 1229 1230 1231
Births
Deaths
... see below for generalizations) with a being non-zero. These solutions are also called the roots of the equation. The formula reads <math> x=\frac{-b \pm \sqrt ...