- 若{an}是等差数列,首项a1>0,a2011+a2012>0,a2011×a2012<0则使前n项和Sn>0成立的最大自然数n是( )A.4021B.4022C.4...
- 在等差数列{an}中,a6+a8=6,则数列{an}的前13项之和为( )A.392B.39C.1172D.78
- 等差数列{an}的前n项和为Sn,若S15为一确定常数,下列各式也为确定常数的是( )A.a2+a13B.a2a13C.a1+a8+a15D.a1a8a15
- 已知a>0,b>0,且2是2a与b的等差中项,则1ab的最小值为( )A.14B.12C.2D.4
- 设A 和G 分别是a,b 等差中项和等比中项,则a2+b2的值为( )A.2A2-G2B.4A2-G2C.2A2-2G2D.4A2-2G2
- 设{an},{bn}分别为等差数列与等比数列,且a1=b1=4,a4=b4=1,则以下结论正确的是( )A.a2>b2B.a3<b3C.a5>b5D.a6>b6
- 设{an}是正项等差数列,{bn}是正项等比数列,且a1=b1,a2n+1=b2n+1则( )A.an+1=bn+1B.an+1≥bn+1C.an+1≤bn+1D.an+1<bn+1
- 若互不相等的实数a,b,c成等差数列,c,a,b成等比数列,且a+3b+c=10,则a=( )A.4B.2C.-2D.-4
- 设等差数列{an}的前n项和为Sn,若S4=8,S8=20,则a11+a12+a13+a14=( )A.18B.17C.16D.15
- 设数列{an}是首项为a1公差为-2的等差数列,如果a1+a4+a7=50,那么a3+a6+a9=( )A.28B.-78C.-48D.38
- 常数列一定是( )A.等差数列B.等比数列C.既是等差数列又是等比数列D.既不是等差数列又不是等比数列
- 等比数列{an}的各项都是正数,且a2,12a3,a1成等差数列,则a4+a5a5+a6的值是( )A.5-12B.1-52C.5+12D.5-12或
- 已知等差数列{an}的前n项和为Sn,若a4=18-a5,则S8=( )A.72B.68C.54D.90
- 设等差数列{an}的前n项和为Sn,若S3=9,S6=36,则a7+a8+a9=( )A.63B.45C.36D.27
- 已知{an}为等差数列,a1+a3+a5=105,a2+a4+a6=99,则a20等于( )A.-1B.1C.3D.7