已知等差数列{An}前n项和Sn.且满足a2=3.S6=36 (1)求数列{An}的通项公式(2)数列{Bn}为等比数列且满足b...
已知等差数列{An}前n项和Sn.且满足a2=3.S6=36
(1)求数列{An}的通项公式(2)数列{Bn}为等比数列且满足b1+b2=3.b4+b5=24.求数列{An乘Bn}的前n项和Tn及Tn大于等于2012时n的最小值
人气:433 ℃ 时间:2020-02-04 06:21:12
解答
a2=a1+d=3
d=3-a1
s6=6a1+15d=6a1+15(3-a1)=45-9a1=36
9a1=9
a1=1
d=2
an=1+2(n-1)=2n-1
b4+b5=b4(1+q)
b1+b2=b1(1+q)
b4/b1=24/3=8
q^3=8
q=2
b1=3/(1+2)=1
bn=2^n-1
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