已知等差数列{an}与等比数列{bn}的前N项和分别为Sn,Tn,且a2=b3=12,a5=b4=18
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已知等差数列{an}与等比数列{bn}的前N项和分别为Sn,Tn,且a2=b3=12,a5=b4=18
1.求an,bn
2.求T6
3.若Sn=190,求n
1.求an,bn
2.求T6
3.若Sn=190,求n
(1)设an公差为d
a5-a2=3d=18-12=6
d=2
a1=a2-d=10
所以an=a1+(n-1)d=2n+8
设bn公比为q
b4/b3=18/12=3/2
b1=b3/q=9
bn=b1q^(n-1)=6×(3/2)^n
(2)T6=b1(1-q^6)/(1-q)
=6×(1-729/64)/(1-3/2)
=6×665/32
=1995/16
(3)Sn=n[2a1+(n-1)d]/2
=n[20+2(n-1)]/2
=n(n+9)
=190
n1=-19(舍去)n2=10
a5-a2=3d=18-12=6
d=2
a1=a2-d=10
所以an=a1+(n-1)d=2n+8
设bn公比为q
b4/b3=18/12=3/2
b1=b3/q=9
bn=b1q^(n-1)=6×(3/2)^n
(2)T6=b1(1-q^6)/(1-q)
=6×(1-729/64)/(1-3/2)
=6×665/32
=1995/16
(3)Sn=n[2a1+(n-1)d]/2
=n[20+2(n-1)]/2
=n(n+9)
=190
n1=-19(舍去)n2=10
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