15题,
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15题,
15题,
15题,
a(n+1)=2an+3
a(n+1)+3=2an+6=2(an+3)
(a(n+1)+3)/(an+3)=2
∴an+3是等比数列
a1+3=1+3=4
∴an+3=4*2^(n-1)
=2^2*2^(n-1)
=2^(n+1)
an=2^(n+1)-3
(2)
稍等