x^2 -4x + m-1= 0
x^2 -4x +4 + m -5 = 0
(x-2)^2 = 5-m
1）5-m < 0 ：无解
2）5－m=0：x=2
3）5-m>0
x-2 = +- (5-m)^1/2
x1 = - (5-m)^1/2 +2
x2= (5-m)^1/2 +2