f(x)=x+2(x>=2)或f(x)=x^2(0<=x<2),当f(f(f(k)))=25/4,k
f(f(f(k)))=25/4 ==> f(f(k)) =25/4 -2 =17/4
==> f(k) =17/4 -2 =9/4
==> k =(9/4)^(1/2) =3/2
f(f(f(k)))=25/4 ==> f(f(k)) =25/4 -2 =17/4
==> f(k) =17/4 -2 =9/4
==> k =(9/4)^(1/2) =3/2