an是等差数列,bn=1/2^an,已知b1+b2+b3=21/8,b1b2b3=1/8,求通向公式an,

{an}是等差数列,设公差为dbn/b(n-1)=(1/2)^an/(1/2)^a(n-1)=(1/2)^[a(n)-a(n-1)]=(1/2)^d则{bn}是等比数列设公比为qb1*b2*b3=1/8所以b2³=1/8 b2=1/2所以(1/2)/q+(1/2)+(1/2)*q=21/8所以q=4或q=1/...