7 8 9 78 789 789 987 789 987 778899 998877 778899 998877 78987 78987 78987 87898 87898 87898 98789 98789 98789 789 798 879 897 978 987 778899 779988 887799 889977 997788 998877
混合练习：78987 87979 87979 88789 78978 79887 98887 78977 79888 78987 99987 78999 77887 78887 99887 77898 89878 97878 98789 97778
