Sn=1+(1+1/2)+(1+1/2+1/4).+(1+1/2+1/4.+1/2^(n-1) )化简

Sn=(2-1)+(2-1/2)+(2-1/4)+.{2-1/2^(n-1)}
=2*n-[1+1/2+1/4+.1/2^(n-1)]
=2n-{2-1/2^(n-1)]
=2n-2+1/2^(n-1)我想问问2-1/2^(n-1)}怎么来的咧？1+1/2+……是个等比数列前n项和啊 首项1公比1/2