题目

在等差数列中，若，，则等于（   ） A. 9                   B. 7                   C. 6                   D. 5 答案：B 【解析】 【分析】 利用等差数列的性质能求出 ，利用等差数列前n项和公式能求出a2=-1，求得d,由此能求出a5． 【详解】因为， 所以5a7=55，所以， 因为，所以 ， 所以公差 ， 所以故选：B． 【点睛】本题考查等差数列的第5项的求法，是基础题，解题时要认真审题，注意等差数列的性质的合理运用用括号里所给词的适当形式填空。每空一词。 1._________________ (woman) Day is my sister’s favourite festival in the year. 2.We need to realize the           (important) of making full use of our time. 3.After lying in bed for three hours, I felt even           (sleepy). 4.Miss Wang likes wearing _______________ (colour) clothes. 5.-- Maths is too difficult. I nearly give it up. -- Please don’t. Nothing is _______________ (possible) if you put your heart into it.