f(x)=3x-1
f(x+1)=3x+2 =3(x+1)-1令t=x+1,则有f(t)=3t-1所以f(x)=3x-1
方法一: 令t=x+1 则x=t-1 f(t)=3(t-1)+2 解得f(t)=3t-1 即:f(x)=3x-1方法二:由f(x+1)=3x+2 得f(x)=3(x+1)-1 令t=x+1,则有 f(t)=3t-1 即:f(x)=3x-1