a1=3,a2=6,a3=6-3=3,a4=3-6=-3,a5=-3-3=-6,a6=-6-(-3)=-3,a7=-3+6=3,a8=3+3=6...可见,每隔着6个为一循环.2009/6=334...5余5,说明a2009=a5=-6
