令t=x-1 ==> 1≤t≤2
x=t+1
x^2=t^2+2t+1
y=(t^2+2t+1)/t
=t+(1/t)+2
函数y=t+(1/t)+2在[1,2]上是增函数,所以
y(max)=y(2)=9/2
y(min)=y(1)=4
值域为：[4,9/2]