作业帮7.8.9.怎么写?过程?
来源：学生作业帮助网 编辑：作业帮 时间：2024/11/29 10:30:10
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7.8.9.怎么写?过程?
7.8.9.怎么写?过程?
7.8.9.怎么写?过程?
7.
设公比为q,则 a5 = a2×q³,即 1/4 = 2×q³,q=1/2,a1 = a2/q = 2×2=4；
a1a2 = 8,a2a3 = a1a2×q² = 1/4 a1a2,a3a4 = a2a3×q² = 1/4 a2a3,.
则 a1a2 + a2a3 + …… + An An+1 = 8×[1-(1/4)^n]/（1-1/4） = 32[1-1/4^n]/3
8.
S1 = A1 = 3 + k
S2 = A1 + A2 = 3 + k + (3+K)q = 9 + k => A2 = (3+K) q = 6
S3 = A1 + A2 + A3 = 3 + k + (3+K)q + (3+K)q^2 = 27 + k => A3 = (3+k)q^2 = 18
q = A3/A2 = 3,3（3+k）=6,k=-1
9.
a2 + a4 + a6 + …… + a100 = q（a1 + a3 + a5 +……＋a99）=1/3×60 = 20
20 + 60 = 80