己知公差大于零的等差数列an,a2＋a3＋a4＝9,且a2＋1,a3＋3,a4＋8为等比数列bn前三项（1）求an,bn

来源：学生作业帮编辑：大师作文网作业帮分类：数学作业时间：2024/11/10 17:17:56

己知公差大于零的等差数列an,a2＋a3＋a4＝9,且a2＋1,a3＋3,a4＋8为等比数列bn前三项（1）求an,bn通项
（2）设an前n项和Sn，求1/S1＋1/S2＋1/Sn

1)假设公差为t
则a3=a2+t
a4=a2+2t
==>3a2+3t=9==>a2+t=3==>a3=3
又a2＋1,a3＋3,a4＋8为等比数列
==》(3-t+1)(3+t+8)=36
==>t=1
==>an=n
==>bn=3*2^(n-1)
2)Sn=[(n+1)n]/2
==>1/S1＋1/S2＋...1/Sn=2/(1*2)+2/(2*3)+...2/[(n+1)n]
又1/[(n+1)n]=1/n-1/(n+1)
==>1/S1＋1/S2＋...1/Sn=2/(1*2)+2/(2*3)+...2/[(n+1)n]
=2[1-1/2+1/2-1/3+1/3-1/4.+n/1-1/(n+1)]
=2[1-1/(n+1)]
=2n/(n+1)

己知公差大于零的等差数列an,a2＋a3＋a4＝9,且a2＋1,a3＋3,a4＋8为等比数列bn前三项（1）求an,bn 已知公差大于零的等差数列an满足a3•a4=48 a2+a5=14 求通项an 若Bn=(根号2）^an 求已知单调递增的等比数列{an}满足a2+a3+a4=28,且a3+2是a2,a4的等差数列,求bn=9+log1/2an an为等差数列,bn为等比数列,a1=b1=1,a2+a4=b3,b2*b4=a3,求an的前10项和及bn 等差数列an公差不等于0且a1,a3,a9成等比数列,求（a1＋a3＋a9）除（a2＋a4＋a10）设{an}为等差数列,且公差d为正数,已知a2+a3+a4=15,又a2,a3-1,a4成等比数列,求a1和d? 设{An}为等差数列,且公差d为正数,已知a2+a3+a4=15,又a2,a3-1,a4成等比数列,求a1和d ｛an｝｛ bn｝分别为等差数列与等比数列且a1=b1=4,a4=b4=1 A.a2大于b2 B.a3小于b3 设{an}为等差数列,{bn}为等比数列,a1=b1=1,a2+a4=b3,b2b4=a3,(1)试求{an}及{bn} 已知数列{an}是公差不为零的等差数列,a1＝1且a1 a2 a4成等比数列,1求通项an 2令bn＝an+2^an求数设{an}未等差数列,{bn}为等比数列,a1=b1=1,a2+a4=b3,b2b4=a3,分别求{an} ,{bn}前设{an}为等差数列,{bn}为等比数列,a1=b1=1,a2+a4=b3,b2b4=a3,分别求出{an}及{bn}的