题目

已知单调递减数列的前项和为，，且，则_____. 答案：【答案】【解析】根据，再写出一个等式：，利用两等式判断并得到等差数列的通项，然后求值.当时，，∴．当时，，①，②①②，得，化简得，或，∵数列是递减数列，且，∴舍去．∴数列是等差数列，且，公差，故．There is no doubt that I'm trying to do what is best, but ________ I have got to consider the cost.A．merelyB．equallyC．nearlyD．mostly