原式
=1/5-1/7+1/7-1/9+1/9-1/11
=1/5-1/11
=6/55怎么算的1/5-1/7>0，则
|(1/5)-(1/7)|=1/5-1/7


1/9-1/7<0，则
|(1/9)-(1/7)|=1/7-1/9

1/9-1/11>0，则
|(1/9)-(1/11)|=1/9-1/11
所以去绝对值得
=1/5-1/7+1/7-1/9+1/9-1/11

然后中间正负抵消得到
=1/5-1/11
通分就得到
=6/55