2分之1+4分之1+8分之1+16分之1···+1024分之1+2048分之1
=1-1/2+1/2-1/4+1/4-1/8+1/8-1/16+.+1/512-1/1024+1/1024-1/2048
=1-1/2048
=2047/2048