1+1/3-7/12+9/20-11/30+13/42-15/56+17/72
=1+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+1/9
=1又1/9