3/4+3/16+3/64+3/256+3/1024+3/4096
=3/4+3/16+3/64+3/256+3/1024+(3/4096+1/4096)-1/4096
=3/4+3/16+3/64+3/256+3/1024+1/1024-1/4096
=3/4+3/16+3/64+3/256+1/256-1/4096
=...自己推下来...
=3/4+1/4-1/4096
=1-1/4096
=4095/4096