kwrl.net
当前位置:首页 >> 等差数列{An}中,A1=2,公差D≠0,且A1,A3,A11恰... >>

等差数列{An}中,A1=2,公差D≠0,且A1,A3,A11恰...

a1, a3, a11在等比 2(2+10d)=(2+2d)² 1+5d=(1+d)² 1+5d=1+2d+d² 3d=d² d=3 故an=a1+d(n-1)=2+3(n-1)=3n-1

(I)设等差数列{an}的公差为d≠0,由题意a1,a11,a13成等比数列,∴a211=a1a13,∴(a1+10d)2=a1(a1+12d),化为d(2a1+25d)=0,∵d≠0,∴2×25+25d=0,解得d=-2.∴an=25+(n-1)×(-2)=-2n+27.(II)由(I)可得a3n-2=-2(3n-2)+27=-6n+31,可...

因为a1^2=a11^2,所以 a1=a11或a1=-a11因为d0 an=a1+(n-1)d=a1+(n-1)*(-a1/5) an=6a1/5-a1n/5>0 所以当n=6时, an=0即第5项和第6项SN最大

∵在等差数列{an}中3a2=7a7,a1>0,∴设公差为d,则3(a1+d)=7(a1+6d),即d=?439a1<0,∴C错误.a11=a1+10d=a1+10(?439a1)=?139a1<0,∴A错误.Sn=na1+n(n?1)2d=na1?n(n?1)2×439a1=a139(?2n2+41n)=?2a139(n?414)2+2a139×(414)2,对应的抛...

a5+2d=a7 a3+a11=2a7 a5+a9=2a7 所以有 5a7=200 a7=40

(Ⅰ)∵等差数列{an}的公差不为零,a1=25,且a1,a11,a13成等比数列,∴(a1+10d)2=a1(a1+12d)d≠0,解得d=-2,∴an=-2n+27.(Ⅱ)∵bn=an+2n,∴Sn--2(1+2+3+…+n)+27n+(2+22+23+…+2n)=-2×n(n+1)2+27n+2(1?2n)1?2=-n2+26n+2n+1-2.

a=7 d=2 n=11 根据an=a1+(n_1)d得 a11=7+(11-1)x2=27

a3a11=(a4+2.5)² (1+2d)(1+10d)=(1+3d+2.5)² 1+12d+20d²=12.25+21d+9d² 11d²-9d-11.25=0 44d²-36d-45=0 (22d+15)(2d-3)=0 因为d>0, 所以2d-3=0, 得d=1.5 故an=a1+d(n-1)=1+1.5(n-1)=1.5n-0.5

a1²+a11²=50 a1²+(a1+10d)²=50 a1²+a1²+20a1d+100d²=50 a1²+10a1d+50d²-25=0 b²-4ac≥0 100d²-4(50d²-25)≥0 100d²-200d²+100≥0 100d²≤100 d²≤1 -1≤d≤1

网站首页 | 网站地图
All rights reserved Powered by www.kwrl.net
copyright ©right 2010-2021。
内容来自网络,如有侵犯请联系客服。zhit325@qq.com