原式 =1-1/2- 1/4 - 1/8- 1/16-1/32-1/64-1/128-1/256
= 1-[1/2+ 1/4 + 1/8+ 1/16+1/32+1/64+1/128+(1/256+1/256)-1/256]
=1-(1-1/256)
= 1/256