1/2+1/4+1/8+1/16+1/32+1/64+1/128,加一个1/128,再减一个1/128
=1/2+1/4+1/8+1/16+1/32+1/64+1/128+1/128-1/128
=1/2+1/4+1/8+1/16+1/32+1/64+1/64-1/128
=1/2+1/4+1/8+1/16+1/32+1/32-1/128
=1/2+1/4+1/8+1/16+1/16-1/128
=1/2+1/4+1/8+1/8-1/128
=1/2+1/4+1/4-1/128
=1/2+1/2-1/128
=1-1/128
=127/128