3/4+3/16+3/64+3/256+3/1024+3/4096＝3/4*[1-（1/4）^6]/(1-1/4)=4095/4096或者3/4+3/16+3/64+3/256+3/1024+3/4096＝3072/4096＋768/4096＋192/4096＋48/4096＋12/4096+3/4096＝4095/4096