General formula of summation formula of proportional series: an = a1× q (n-1);

Let bn=an+3.

It is a geometric series BN = 2 (n-1) b1= 2 (n-1) (a1+3) with a common ratio of 2.

Because a(n+ 1)=2an+3, let a(n+ 1)+λ=2(an+λ) calculate λ=3, we need to use a new bn series, let bn=an+λ=an=3.

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