綾 5個月以前 公比為-1/2 -1/32 = 16*(-1/2)^9 -1/32為第9項 代等比級數公式得 S9 = 16[1-(-1/2)^9] / [1-(-1/2)] = [16 - (-1/2^5)] / (3/2) = (32 + 1/16)/3 = 513/48 留言