令n=1/2+1/4+1/8+1/16+······+1/2048，则2n=1+1/2+1/4+1/8+1/16+······+1/1024n=2n-n=1－1/2048=2047/2048
后面借个1／2048，算出答案再减就是了。你自己做做看
