b) Phép tính thích hợp là: 

Đúng ghi Đ, sai ghi S.

a) \[1 - \frac{2}{5} = \frac{3}{5}\]                          Đ   

b) \[\frac{{40}}{{45}} - \frac{2}{9} = \frac{7}{9}\]                    S

c) \[\frac{{17}}{9} - \frac{{19}}{{18}} = \frac{5}{6}\]               Đ                

d) \[3 - \frac{{12}}{5} < 1\]                           Đ

Giải thích

a) \[1 - \frac{2}{5} = \frac{5}{5} - \frac{2}{5} = \frac{3}{5}\]                      

b) \[\frac{{40}}{{45}} - \frac{2}{9} = \frac{{40}}{{45}} - \frac{{10}}{{45}} = \frac{{30}}{{45}} = \frac{2}{3}\]                 

c) \[\frac{{17}}{9} - \frac{{19}}{{18}} = \frac{{34}}{{18}} - \frac{{19}}{{18}} = \frac{{15}}{{18}} = \frac{5}{6}\]              

d) \[3 - \frac{{12}}{5} = \frac{{15}}{5} - \frac{{12}}{5} = \frac{3}{5} < 1\] do 3 < 5.

d) Đáp án đúng là: C

\[\frac{9}{5} - \frac{{21}}{{15}} = \frac{{27}}{{15}} - \frac{{21}}{{15}} = \frac{6}{{15}} = \frac{2}{5}\]

\[\frac{2}{3} - \frac{1}{{15}} = \frac{{10}}{{15}} - \frac{1}{{15}} = \frac{9}{{15}} = \frac{3}{5}\]

\[\frac{{32}}{{15}} - \frac{9}{5} = \frac{{32}}{{15}} - \frac{{27}}{{15}} = \frac{5}{{15}} = \frac{1}{3}\]

\[\frac{3}{5} - \frac{7}{{15}} = \frac{9}{{15}} - \frac{7}{{15}} = \frac{2}{{15}}\]

Vậy phép tính có kết quả bằng \[\frac{1}{3}\] là: \[\frac{{32}}{{15}} - \frac{9}{5}\]