已知数列(an中,a1=3.a2=6.an+2=an+1_an则a2009

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