1.f（x）=x²,得f（x-1）=（x-1）^2=x^2-2x+1
2.f(x-1)=x^2=(x-1)^2+2(x-1)+1得,f(x)=x^2+2x+1
3.设f(x)=ax+b 则f(f(x))=f(ax+b)=a(ax+b)+b=a^2x+ab+b=9x+4
则有,a^2=9,ab+b=4,解得a=3,b=1,或a=-3,b=-2
f(x)=3x+1或f(x)=-3x-2
1.f（x）=x²,得f（x-1）=（x-1）^2=x^2-2x+1
2.f(x-1)=x^2=(x-1)^2+2(x-1)+1得,f(x)=x^2+2x+1
3.设f(x)=ax+b 则f(f(x))=f(ax+b)=a(ax+b)+b=a^2x+ab+b=9x+4
则有,a^2=9,ab+b=4,解得a=3,b=1,或a=-3,b=-2
f(x)=3x+1或f(x)=-3x-2