n =b1.q^(n-1)
bn=an-3n
an = bn+3n
=b1.q^(n-1) + 3n
Sn =a1+a2+...+an
=b1(q^n-1)/(q-1) + 3n(n+1)/2
已知数列an满足bn=an-3n,且bn为等比数列,求an前n项和Sn
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