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a(n+1) = 3a(n) - 4n+ 2 = 3a(n) - 6n + 2(n+1),
a(n+1) - 2(n+1) = 3[a(n) - 2n],
{a(n)-2n}是首项为a(1)-2=-1,公比为3的等比数列.
a(n) - 2n = (-1)3^(n-1),
a(n) = 2n - 3^(n-1),
s(n) = n(n+1) - [3^n - 1]/(3-1)= n(n+1) - [3^n - 1]/2
1年前
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