Centuries: 12th century - 13th century - 14th century

Decades: 1230s 1240s 1250s 1260s 1270s - 1280s - 1290s 1300s 1310s 1320s 1330s

Years: 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289

... is derived by the method of completing the square[?]. <math>ax^2+bx+c=0</math> Dividing our quadratic equation by a, we have <math> x^2 + \left( ...