染在睫毛上的阳光
幼苗
共回答了23个问题采纳率:87% 举报
(1)
an=a1+(n-1)d, Sn=a1=a2+...+an
S1.S4 =(S2)^2
2a1(2a1+3d) = (2a1+d)^2
6a1d=4a1d+d^2
d(2a1-d)=0
a1=d/2
S1=a1=d/2
S2 = 2a1+d = 2d
S2/S1 = (2d)/(d/2) = 4
公比=4
(2)
S2=4
2d=4
d=2
a1=1
an = 1+2(n-1) = 2n-1
(3)
bn=3/[an.a(n+1)]
Tn=b1+b2+...+bn
Tn
1年前
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