问题
选择题
已知等差数列{an}的前n项和为Sn，若a4+a5=18，则S8等于（ ）
A．18
B．36
C．54
D．72
答案
∵数列{an}为等差数列，a4+a5=18，
∴由等差数列的性质得：a4+a5=a1+a8=18，
又其前n项和为Sn，
∴S8=
=72．(a1+a8)×8 2
故选D．
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已知等差数列{an}的前n项和为Sn，若a4+a5=18，则S8等于（ ）
A．18
B．36
C．54
D．72
∵数列{an}为等差数列，a4+a5=18，
∴由等差数列的性质得：a4+a5=a1+a8=18，
又其前n项和为Sn，
∴S8=
=72．(a1+a8)×8 2
故选D．
书面表达(满分25分) 党中央号召要创建节约型社会，可是许多校园内部浪费现象还比较严重。请你在班会上发表演讲，号召同学们从我做起，改变不良习惯，杜绝浪费。
Dear friends, May I have your attention, please? Now I’d like to make a speech here. Nowadays, some of us students have formed some bad habits on campus. For example, _______________________________________________________ _________________________________________________________________________ _______________________________________________________________________  ____But how to reduce the waste? In my opinion, _______________________________ ____________________________________________________________________________ ___________________________________________________________________________ ____________________________________________________________________________ That’s all. Thank you! |