原式=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)
=10/9
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=10/9
1+1/3-7/12+9/20-11/30+13/42-15/56+17/72=(4)
1+1/3-7/12+9/20-11/30+13/42-15/56+17/72=(10/9 )