因为1/4+1/8+1/16+1/32+1/64+1/128+1/256+1/512
=(1/2+1/4+1/8+1/16+1/32+1/64+1/128+1/256+1/512)-1/2
=1-1/512-1/2
=1/2-1/512
=255/512