f(x)=ax+b3f(x+1)-2f(x-1)=3[a(x+1)+b]-2[a(x-1)+b]=3ax+3a+3b-2ax+2a-2b=ax+5a+b=2x+17既ax=2x,5a+b=17a=2,b=7f(x)=2x+7
