1+1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90+1/110
=1+(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+...+(1/10-1/11)
=1+(1-1/11)
=2-1/11
=21/11