1＋1／2＋3／4＋7／8＋15／16＋31／32＋63／64＋127／128
=1+1-1/2+1-1/4+1-1/8+1-1/16+1-1/32+1-1/64+1-1/128
=8-(1/2+1/4+1/8+1/16+1/32+1/64+1/128)
=8-(1+2+4+.+64)/128
=8-127/128
=7又1/128