1/2-1/6-1/12-1/20-1*30-1/42-1/56-1/72-1/90=1/2-(1/2-1/3)-(1/3-1/4)-(1/4-1/5)-(1/5-1/6)-(1/6-1/7)-(1/7-1/8)-(1/8-1/9)-(1/9-1/10)=1/10
