设公比为q
-1=(-16)q⁴
q⁴=1/16
q=1/2或q=-1/2
q=1/2时,a1=(-16)q=-8 a2=a1q=-4 a3=a2q=-2 a1+a2+a3=(-8)+(-4)+(-2)=-14
q=-1/2时,a1=(-16)q=8 a2=a1q=-4 a3=a2q=2 a1+a2+a3=8+(-4)+2=6
设公比为q
-1=(-16)q⁴
q⁴=1/16
q=1/2或q=-1/2
q=1/2时,a1=(-16)q=-8 a2=a1q=-4 a3=a2q=-2 a1+a2+a3=(-8)+(-4)+(-2)=-14
q=-1/2时,a1=(-16)q=8 a2=a1q=-4 a3=a2q=2 a1+a2+a3=8+(-4)+2=6