y=f(x)=x^2-3x+2=(x-3/2)^2-1/4
x<3/2,所以y>-1/4
y+1/4=(x-3/2)^2
x<3/2,|x-3/2|=3/2-x
√(y+1/4)=√(x-3/2)^2=3/2-x
x=3/2-√(y+1/4)
所以反函数是y=3/2-√(x+1/4),x>-1/4