f(n)=cos(nπ/2+π/4) 则有
f(n)=f(n+4)
f(1)=-1/2 f(2)=-1/2 f(3)=1/2 f(4)=1/2
f(1)+f(2)+f(3)+f(4)=0
则有 f(1)+f(2)+……+f(2000)=0