f[f(x)]=9X+3 求f(X)的值

f(x) = ax + b
f(f(x)) = f(ax+b) = a(ax+b) + b = a^2x + ab + b = 9x + 3
a^2 = 9 ---> a = 3, a = -1
a = 3 ---> 3b + b = 3 ---> b = 3/4
a = -3 ---> -3b + b = 3 ---> b = -3/2

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设f(X)=ax+b,再带入f[f(x)]，整理，求得。