题目上有点问题,那个/1应该是没有的

原式=1/(1*3)+1/(3*5)+……+1/(9*11)
=2*[1/(1*3)+1/(3*5)+……+1/(9*11)]/2
=[2/(1*3)+2/(3*5)+……+2/(9*11)]/2
=(1-1/3+1/3-1/5+1/5-1/7……+1/9-1/11）/2
=（1-1/11）/2
=5/11