设f(x)=ax+b
则:f(f(x))=a(ax+b)+b=a^2x+ab+b
f(f(f(x))=a^2(ax+b)+ab+b=a^3x+a^2b+ab+b
f{ f[ f(x)]}=27x+52
所以,a^3=27,a^2b+ab+b=52
a=3,b=4
f(x)=3x+4
2)
f(x)=3x+4=0
x=-4/3
3)
f(x)=3x+4>0
x>-4/3
设f(x)=ax+b
则:f(f(x))=a(ax+b)+b=a^2x+ab+b
f(f(f(x))=a^2(ax+b)+ab+b=a^3x+a^2b+ab+b
f{ f[ f(x)]}=27x+52
所以,a^3=27,a^2b+ab+b=52
a=3,b=4
f(x)=3x+4
2)
f(x)=3x+4=0
x=-4/3
3)
f(x)=3x+4>0
x>-4/3