a) 8 + 4 = 12               15 – 6 = 9                9 + 2 = 11                 6 + 8 = 14

   4 + 8 = 12                15 – 9 = 6               11 – 9 = 2                  14 – 8 = 6

b)

\(\begin{array}{*{20}{c}}{ + \begin{array}{*{20}{c}}{48}\\{27}\end{array}}\\\hline{\,\,\,75}\end{array}\)                             \(\begin{array}{*{20}{c}}{ + \begin{array}{*{20}{c}}{25}\\{75}\end{array}}\\\hline{\,100}\end{array}\)                                \(\begin{array}{*{20}{c}}{ - \begin{array}{*{20}{c}}{80}\\{\,\,7}\end{array}}\\\hline{\,\,\,73}\end{array}\)                          \(\begin{array}{*{20}{c}}{ - \begin{array}{*{20}{c}}{100}\\{\,\,\,\,\,8}\end{array}}\\\hline{\,\,\,\,\,\,92}\end{array}\)

\(\begin{array}{*{20}{c}}{ - \begin{array}{*{20}{c}}{82}\\{55}\end{array}}\\\hline{\,\,\,27}\end{array}\)                             \(\begin{array}{*{20}{c}}{ - \begin{array}{*{20}{c}}{50}\\{39}\end{array}}\\\hline{\,\,\,11}\end{array}\)                                \(\begin{array}{*{20}{c}}{ + \begin{array}{*{20}{c}}{91}\\{\,\,9}\end{array}}\\\hline{100}\end{array}\)                          \(\begin{array}{*{20}{c}}{ - \begin{array}{*{20}{c}}{100}\\{\,\,28}\end{array}}\\\hline{\,\,\,\,\,72}\end{array}\)

c) 42 + 18 – 10 = 60 – 10 = 50

   60 – 13 + 23 = 47 + 23 = 70

2 x 2 = 4

2 x 7 = 14

2 x 3 = 6

2 x 8 = 16

2 x 4 = 8

2 x 9 = 18

\(\begin{array}{*{20}{c}}{a)\,\,}\\{}\\{}\end{array}\begin{array}{*{20}{c}}{ + \begin{array}{*{20}{c}}{64}\\{36}\end{array}}\\\hline{\,\,100}\end{array}\)                                \(\begin{array}{*{20}{c}}{ + \begin{array}{*{20}{c}}{79}\\{21}\end{array}}\\\hline{\,100}\end{array}\)

       \(\begin{array}{*{20}{c}}{ + \begin{array}{*{20}{c}}{52}\\{48}\end{array}}\\\hline{\,\,100}\end{array}\)                                 \(\begin{array}{*{20}{c}}{ + \begin{array}{*{20}{c}}{34}\\{66}\end{array}}\\\hline{\,100}\end{array}\)

b,

a)

\(\begin{array}{*{20}{c}}{ - \begin{array}{*{20}{c}}{100}\\{\,\,19}\end{array}}\\\hline{\,\,\,\,\,81}\end{array}\)                                      \(\begin{array}{*{20}{c}}{ - \begin{array}{*{20}{c}}{100}\\{\,\,66}\end{array}}\\\hline{\,\,\,\,\,34}\end{array}\)

\(\begin{array}{*{20}{c}}{ - \begin{array}{*{20}{c}}{100}\\{\,\,75}\end{array}}\\\hline{\,\,\,\,\,\,25}\end{array}\)                                      \(\begin{array}{*{20}{c}}{ - \begin{array}{*{20}{c}}{100}\\{\,\,94}\end{array}}\\\hline{\,\,\,\,\,\,\,\,6}\end{array}\)

b)

Ta có:

9 + 8 = 17        8 + 4 = 12          8 + 8 = 16

9 + 6 = 15       9 + 4 = 13          8 + 6 = 14

Từ phép nhân 2 × 3 = 6; em lập được 2 phép chia tương ứng: 6 : 2 = 3 và 

6 : 3 = 2

Từ phép nhân 2 × 6 = 12; em lập được 2 phép chia tương ứng: 12 : 2 = 6 và 

12 : 6 = 2

Từ phép nhân 2 × 9 = 18; em lập được 2 phép chia tương ứng: 18 : 2 = 9 và 

18 : 9 = 2