1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90+1/110
=1/1*2+1/2*3+……+1/10*11
=(1/1-1/2)+(1/2-1/3)+……+(1/10-1/11)
=1/1-1/11
=10/11