设S=1/2+2/4+3/8+4/16+5/32+6/64+7/128+8/256+9/512+10/1024
2S=1+2/2+3/4+...+10/512
2S-S=1+1/2+1/4+1/8+...+1/512-10/1024=S
S=1+511/512-10/1024
=509/256