x2 - 3x - 5 = 0 整理,得：x2 - 3x = 5 两边同时加上一次项系数一半的平方,得：x2 - 3x + (3/2)2 = 5 + (3/2)2 整理,得：(x - 3/2)2 = 29/4 两边开平方,得：x - 3/2 = ±√29/2 = 3+√29/2或x?= 3-√29/2