2/5 (1/2-1/3+1/3-1/4……+1/9-1/10=1/2-1/10)
3/10 [1/3(1-1/4+1/4-1/7+1/7-1/10)]=1/3*9/10(题目错了吧~)
4/9 [1/2(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9)]=1/2*8/9
1/10 1/5-1/6+1/6-1/7+……+1/9-1/10=1/5-1/10 (题目有错了吧,每个乘号应该隔着一个加号~~)
2/5 (1/2-1/3+1/3-1/4……+1/9-1/10=1/2-1/10)
3/10 [1/3(1-1/4+1/4-1/7+1/7-1/10)]=1/3*9/10(题目错了吧~)
4/9 [1/2(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9)]=1/2*8/9
1/10 1/5-1/6+1/6-1/7+……+1/9-1/10=1/5-1/10 (题目有错了吧,每个乘号应该隔着一个加号~~)