等比数列｛an｝中,a2=2,a5=128a5=a2*q^3128=2*q^3q^3=64q=4,a1=a2/q=1/2an=(1/2)*4^(n-1)bn=log2an=log2[(1/2)*4^(n-1)]=2n-3,数列｛bn｝是等差数列b1=-1,d=2数列｛bn｝前n项和Sn=（-1+2n-3)n/2=n^2-2n...