题目其实就是告诉你周期是4，所以f(5)=-5f(-5)=f(3)=-1/5
由f(x+2)=1/f(x)则f(x+4)=1/f(x+2）=1/[1/f(x)]=f(x)则f(x)是周期为4的函数。若f(1)=-5f(5)=f(1)=-5则f(f(5))=f(-5)=f(-5+2*4)=f(3)=f(1+2)=1/f(1)=-1/5
