1/3-7/12+9/20-11/30+13/42-15/56+17/72-19/90+21/110
问题描述：
1/3-7/12+9/20-11/30+13/42-15/56+17/72-19/90+21/110
答
1/3-7/12+9/20-11/30=
1/3-(3+4)/(3*4)+(5+4)/(5*4)+)-(5+6)/(5*6)=
1/3-1/3-1/4+1/4+1/5-1/5-1/6
我列出一部分，其他一样分解，最后消除掉就得结果
答
1/3-7/12+9/20-11/30+13/42-15/56+17/72-19/90+21/110=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+1/11)=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...