1/2-1/6-1/12-1/20-……-1/90 =1/2-(1/2-1/3)-(1/3-1/4)-(1/4-1/5)-……-(1/9-1/10) =1/2-1/2+1/3-1/3+1/4-1/4+1/5-……-1/9+1/10 =1/10
