1/2+5/6+11/12+19/20+ ---- +89/90+109/110

1/2+5/6+11/12+19/20+……+89/90+109/110 =1-1/2+1-1/6+1-1/12……+1-1/110 =10-(1/2+1/6+1/12……+1/110) =10-(1/2+1/2-1/3+1/3-1/4+1/5-1/5……+1/10-1/11) =10-10/11 =100/11