The general term formula an = 3N + 1 of sequence {an} is known. The second term, the fourth term and the eighth term are taken out in turn. The second ^ n term constitutes a new sequence {BN}, Find the general formula of {BN}

The general term formula an = 3N + 1 of sequence {an} is known. The second term, the fourth term and the eighth term are taken out in turn. The second ^ n term constitutes a new sequence {BN}, Find the general formula of {BN}

b1=a2=3×2 + 1
b2=a4=3×2²+ 1
b3=a8=3×2³+ 1
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bn=a(2^n)=3×2^n + 1