已知f(x)=x 2(x>=2)或f(x)=x^2(0<=x<2),当f(f(f(k)))=25/4，k是多少
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
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