a(n+1)=an +a(n+2)a(n+2)=a(n+1)-an a(n+3)=a(n+2)-a(n+1) a(n+2)+a(n+3)=a(n+2)-ana(n+3)=-ana(n+6)=-a(n+3)=an，数列是以6为周期的循环数列2015÷6=335余5a2015=a5=-a2=-6
