原式=(1/2+1/4+...+1/2048+1/4096+1/4096)-1/4096=(1/2+1/4+...+1/1024+1/2048+1/2048)-1/4096=(1/2+1/4+...+1/1024+1/1024)-1/4096=...=(1/2+1/2)-1/4096=1-1/4096=4095/4096