f(x+2)=1/f(x),
f(5)= f(3+2)=1/f(3) = 1/f(1+2) =1/[1/f(1)]=f(1)=-5
f(x)=1/f(x+2),
f(f(5))=f(-5)=1/f(-5+2)=1/f(-3)=f(-1)=1/f(1)=-1/5