1/2+1/4+1/8+1/16+.+1/256
=1/2+1/4+1/8+1/16+.+1/256+1/256-1/256(加最后一个数再减最后一个数)
=1/2+1/4+1/8+1/16+.+1/128+1/128-1/256
=1/2+1/4+1/8+1/16+.+1/64+1/64-1/256
.
=1/2+1/2-1/256
=1-1/256
=255/256
1/2+1/4+1/8+1/16+.+1/256
=1/2+1/4+1/8+1/16+.+1/256+1/256-1/256(加最后一个数再减最后一个数)
=1/2+1/4+1/8+1/16+.+1/128+1/128-1/256
=1/2+1/4+1/8+1/16+.+1/64+1/64-1/256
.
=1/2+1/2-1/256
=1-1/256
=255/256