xinghesheng
幼苗
共回答了26个问题采纳率:92.3% 举报
1
Sn=-2n^2-n
S(n-1)=-2(n-1)^2-(n-1)
=-2n^2+4n-2-n+1
=-2n^2+3n-1
Sn-S(n-1)=an=-2n^2-n+2n^2-3n+1
=-4n+1
2
∵an=-4n+1
a(n-2)=-4(n-2)+1
=-4n+8+1
=-4n+9
an-a(n-2)=-4n+1 +4n-9
=-8
∴数列{a(2n-1)}是等差数列,公差d=-8
a1=-4+1=-3
25=2n-1
n=13
∴a1+a3+.+a25
=b1+b2+.b13
=S13=13a1+13x12xd/2
=13x(-3)+12x6x(-8)
=-39-576
=-615
1年前
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