7. (1+x)/x=1+1/xf(1+1/x)=1+1/x²+1/x=(1+1/x)²-1/x=(1+1/x)²-(1+1/x)+1∴f(x)=x²-x+1，选C8.将x用3-x代，得到f(3-x)+2f[3-(3-x)]=(3-x)²，即f(3-x)+2f(x)=(3-x)²，该式乘以2减去原式得4f(x)-f(x)=2(3-x)²-x²，f(x)=x²/3-4x+6，选B
