已知{a
n }是a
1=23,公差d为整数的等差数列,且前6项为正,第7项开始为负.
(1)求d的值;
(2)求前n项之和S
n 的最大值;
(3)当S
n 是正数时求n的最大值.
考点分析:
相关试题推荐
将下列命题改写成“若p则q”的形式,并写出其逆命题、否命题、逆否命题.
(1)正数a的平方根不等于0;
(2)两条对角线不相等的平行四边形不是矩形.
查看答案
在空间四边形ABCD中,AC和BD为对角线,G为△ABC的重心,E是BD上一点,BE=3ED,以{
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281017_ST/0.png)
,
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281017_ST/1.png)
,
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281017_ST/2.png)
}为基底,则
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281017_ST/3.png)
=
.
查看答案
过点M(3,-1)且被点M平分的双曲线
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281016_ST/0.png)
的弦所在直线方程为
.
查看答案
已知向量{
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281015_ST/0.png)
、
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281015_ST/1.png)
,
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281015_ST/2.png)
}是空间的一个基底,从
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281015_ST/3.png)
、
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281015_ST/4.png)
、
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281015_ST/5.png)
中选择向量
,可以与向量P=
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281015_ST/6.png)
-2
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281015_ST/7.png)
,q=
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281015_ST/8.png)
+2
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131101225344155728172/SYS201311012253441557281015_ST/9.png)
构成空间的一个基底.
查看答案
命题“若a,b都是奇数,则a+b是偶数”的逆否命题是
.
查看答案