设Sn是等差数列{an}的前n项和,已知1/3S3与1/4S4的等比中项为1/5S5,1/3S3与1/4S4的等差中项为1.设Sn是等差数列{an}的前n项和,已知1/3S3与1/4S4的等比中项为1/5S5,1/3S3与1/4S4的等差中项为1,求等差数列{an}的
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![设Sn是等差数列{an}的前n项和,已知1/3S3与1/4S4的等比中项为1/5S5,1/3S3与1/4S4的等差中项为1.设Sn是等差数列{an}的前n项和,已知1/3S3与1/4S4的等比中项为1/5S5,1/3S3与1/4S4的等差中项为1,求等差数列{an}的](/uploads/image/z/14299819-43-9.jpg?t=%E8%AE%BESn%E6%98%AF%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%2C%E5%B7%B2%E7%9F%A51%2F3S3%E4%B8%8E1%2F4S4%E7%9A%84%E7%AD%89%E6%AF%94%E4%B8%AD%E9%A1%B9%E4%B8%BA1%2F5S5%2C1%2F3S3%E4%B8%8E1%2F4S4%E7%9A%84%E7%AD%89%E5%B7%AE%E4%B8%AD%E9%A1%B9%E4%B8%BA1.%E8%AE%BESn%E6%98%AF%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%2C%E5%B7%B2%E7%9F%A51%2F3S3%E4%B8%8E1%2F4S4%E7%9A%84%E7%AD%89%E6%AF%94%E4%B8%AD%E9%A1%B9%E4%B8%BA1%2F5S5%2C1%2F3S3%E4%B8%8E1%2F4S4%E7%9A%84%E7%AD%89%E5%B7%AE%E4%B8%AD%E9%A1%B9%E4%B8%BA1%2C%E6%B1%82%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%E7%9A%84)
设Sn是等差数列{an}的前n项和,已知1/3S3与1/4S4的等比中项为1/5S5,1/3S3与1/4S4的等差中项为1.设Sn是等差数列{an}的前n项和,已知1/3S3与1/4S4的等比中项为1/5S5,1/3S3与1/4S4的等差中项为1,求等差数列{an}的
设Sn是等差数列{an}的前n项和,已知1/3S3与1/4S4的等比中项为1/5S5,1/3S3与1/4S4的等差中项为1.
设Sn是等差数列{an}的前n项和,已知1/3S3与1/4S4的等比中项为1/5S5,1/3S3与1/4S4的等差中项为1,求等差数列{an}的通项an
设Sn是等差数列{an}的前n项和,已知1/3S3与1/4S4的等比中项为1/5S5,1/3S3与1/4S4的等差中项为1.设Sn是等差数列{an}的前n项和,已知1/3S3与1/4S4的等比中项为1/5S5,1/3S3与1/4S4的等差中项为1,求等差数列{an}的
Sn=na1 + [n(n-1)d]/2
1/3S3=1/3{3a1 + [3×(3-1)]d/2} = 1/3(3a1 + 3d)= a1 + d
1/4S4=1/4{4a1 + [4×(4-1)]d/2} = 1/4(4a1 + 6d)= a1 + 3d/2
1/5S5=1/5{5a1 + [5×(5-1)]d/2} = 1/5(5a1 + 10d)= a1 + 2d
1/3S3与1/4S4的等比中项为1/5S5,
(a1 + 2d)^2 =(a1 + d)×(a1 + 3d/2)
3a1d/2 + 5d^2/2 = 0 .(1)
1/3S3与1/4S4的等差中项为1,
(a1 + d)+(a1 + 3d/2)=2×1
a1 + 5d/4 = 1.(2)
(1),(2)两式联立,解得:d=0或d=-12/5
当d=0时,代入(2)中,a1=1 ,an=1
当d=-12/5时,代入(2)中,a1=4 ,an=32/5 - 12n/5