1-5/6+7/12-9/20+11/30-13/42+15/56-17/72+19/90 =1-(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) =1-1/2+1/10 =3/5