f(x+1/x)=x^3+1/x^3
=[x+(1/x)][x^2+(1/x)^2-1]
=[x+(1/x)][(x+1/x)^2-2-1]
=[x+(1/x)][(x+1/x)^2-3]
设t=x+(1/x)所以原式f(t)=t(t^2-3)=t^3-3t
所以函数解析式f(x)=x^3-3x