f(x+1)=x^2-3*x+2
f(x)=(x-1)^2-3(x-1)+2=x^2-5x+6,x28/9
f(x)=(x-5/2)^2-1/4=y
y+1/4=(x-5/2)^2
x-5/2=-√（y+1/4）(因为x-5/228/9