馨儿菇凉:1/22+1/30+1/42+1/56+1/72+1/90=?简便方法是什么?

来源：百度文库编辑：杭州交通信息网时间：2024/04/30 02:33:39

1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90
= 1/(4*5) + 1/(5*6) + 1/(6*7) + 1/(7*8) + 1/(8*9) + 1/(9*10)
= (1/4 - 1/5) + (1/5 - 1/6) + .... + (1/9 - 1/10)
= 1/4 - 1/10
= 0.15

写错了吧?
1/20+1/30+1/42+1/56+1/72+1/90
=1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10
=1/4-1/10
=3/20

1/20 + 1/30 + ¼2 + 1/56 + 1/72 + 1/90
= 1/(4*5) + 1/(5*6) + 1/(6*7) + 1/(7*8) + 1/(8*9) + 1/(9*10)
= (¼ - 1/5) + (1/5 - 1/6) + .... + (1/9 - 1/10)
= ¼ - 1/10
= 0.15

1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90
= 1/(4*5) + 1/(5*6) + 1/(6*7) + 1/(7*8) + 1/(8*9) + 1/(9*10)
= (1/4 - 1/5) + (1/5 - 1/6) + .... + (1/9 - 1/10)
= 1/4 - 1/10
= 0.15

1/22+1/30+1/42+1/56+1/72+1/90=?简便方法是什么? 1/30+1/42+1/72+1/90+…1/9900 巧算 1/30+1/42+1/56+1/72+1/90 1/2+1/6+1/12+1/20+1/30 30/1+42/1+56/1+72/1+90/1=? 1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90= 1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90=? 20/1+30/1+42/1+56/1+72/1+90/1 1/20+1/30+1/42+1/56+1/72+1/90 1/2,1/6,1/12,(),1/30