等比数列an的前N项和为Sn,a3=-12,S3=-9,求公比q和S5

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等比数列an的前N项和为Sn,a3=-12,S3=-9,求公比q和S5等比数列an的前N项和为Sn,a3=-12,S3=-9,求公比q和S5等比数列an的前N项和为Sn,a3=-12,S3=-9,求公

等比数列an的前N项和为Sn,a3=-12,S3=-9,求公比q和S5
等比数列an的前N项和为Sn,a3=-12,S3=-9,求公比q和S5

等比数列an的前N项和为Sn,a3=-12,S3=-9,求公比q和S5
设等比数列首项为a1 等比为q 由题意有
a1q^2=-12
a1+a1q+a1q^2=-9
(q+2)^2=0
q=-2
a1=-3
S5=a1(1-q^n)/(1-q)=-33

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