=(1/2-1/4)+(1/2-1/8)+(1/2-1/16)+...+(1/2-1/1024)
=1/2*9-(1/4+1/8+1/16+...+1/1024)
=1/2*10-(1/2+1/4+1/8+...+1/1024)
=5-(1-1/1024)=4+1/1024
=4又1/1024