1-1/2-1/6-1/12-1/20-.-1/9900
=1-1/2-(1/2-1/3)-(1/3-1/4)-(1/4-1/5)-……-(1/99-1/100)
=1-1/2-1/2+1/3-1/3+1/4-1/4+1/5-……-1/99+1/100
=1-1/2-1/2+1/100
=1/100