1/12+1/20+1/30+1/42+1/56+1/72+1/90=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=1/3-1/10=10/30-3/30=7/30。不懂可追问，满意点击右下角采纳，谢谢！
