f(x)=f(x-1)-f(x-2) 令x=x-1，则f(x-1)=f(x-1-1)-f(x-1-2) 即f(x-1)=f(x-2)-f(x-3) 将其代入原式，：f(x)=f(x-2)-f(x-3)-f(x-2) =-f(x-3)即f(x)=-f(x-3)令x=x-3，则f(x-3)=-f(x-3-3)即f(x-3)=-f(x-6)f(x)=-f(x-3)=f(x-6)周期T=6
