an等差数列 bn前n项和sn满足sn=3(bn-1)/2 且a2=b1 a5=b2 ⑴求an bn通项 ⑵设tn为数列sn的前n项和,求tn
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an等差数列 bn前n项和sn满足sn=3(bn-1)/2 且a2=b1 a5=b2 ⑴求an bn通项 ⑵设tn为数列sn的前n项和,求tn
an等差数列 bn前n项和sn满足sn=3(bn-1)/2 且a2=b1 a5=b2 ⑴求an bn通项 ⑵设tn为数列sn的前n项和,求tn
an等差数列 bn前n项和sn满足sn=3(bn-1)/2 且a2=b1 a5=b2 ⑴求an bn通项 ⑵设tn为数列sn的前n项和,求tn
1.Sn=3/2bn-3/2
S(n-1)=3/2b(n-1)-3/2
bn=Sn-S(n-1)=3/2bn-3/2b(n-1)
bn=3b(n-1)
所以{bn}是等比数列,公比3
而b1=S1=3/2b1-3/2
b1=3
所以bn=3^n
a2=b1=3,a5=b2=9
d=(a5-a2)/3=2
an=2n-1
2.Sn=3/2*3^n-3/2
Tn=9/2(1-3^n)/(1-3)-3/2n=9/4(3^n-1)-3/2n
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