数列是等比数列,a4·a5=a1·a8=a2·a7=a3·a6
log2(a1)+log2(a2)+...+log2(a8)
=log2(a1·a2·...·a8)
=log2[(a1·a8)·(a2·a7)·(a3·a6)·(a4·a5)]
=log2[(a4·a5)^4]
=4log2(a4·a5)
=4log2(16)
=4×4
=16