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共回答了26个问题 举报
Sn - S(n-6) = 324 - 144
a(n-5) + a(n-4) + a(n-3) + a(n-2) + a(n-1) + an = 180
a1 + a2 + a3 + a4 + a5 + a6 = 36
两式子相减
(a(n-5) - a1) + (a(n-4) - a2) + …… + (an - a6) = 180 - 36
(n-6)d + (n-6)d + (n-6)d + (n-6)d + (n-6)d + (n-6)d = 144
6(n-6)d = 144
(n-6)d = 24
S6 = 36
6a1+15d = 36
2a1 = 12 - 5d
Sn = 324
[2a1 + (n-1)d] n /2 =324
[12 + (n-6)d] n = 648
(12 + 24)*n = 648
n = 648/36 = 18
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