f(x+1)=x方-3x+2
换元法,设x+1=t ,则有 x =t-1 ,
则 f(x+1)=f(t)=x方-3x+2=(t-1)^2-3(t-1)+2 展开得
f(t) = t^2-2t+1-3t+3+2
= t^2-5t+6