令a=x+1/x
a²=x²+2+1/x²
x²+1/x²=a²-2
x³+1/x³
=(x+1/x)(x²-1+1/x²)
=a(a²-2-1)
=a³-3a
所以f(a)=a³-3a
f(x)=x³-3x