设此等比数列的首项,公比分别为a1,q则由2a7 =a1+a4有2q^6=1+q^3q^3=1或q^3=-1/2 所以当q=1时,此等比数列的前n项和为S3=3a1,2S3=6a1S6=6a1S12-S6=6a1显然此时2倍S3 S6 S12-S6成等比数列 当q^3=-1/2 时,由等比数列的前...