(1) an=a1q^(n-1)bn=loga1+(n-1)log(q)为等差数列(2) b3=6/3=2b1b5=0b1=loga1>0所以b5=0, b1=4所以bn=5-nSn=5n-1/2n^2+1/2n=-1/2n^2+9/2nan=2^(5-n)
