Đúng ghi Đ, sai ghi S. 2/23 + 8/23 = (2+8)/23 = 10/23

a) \[\frac{2}{{23}} + \frac{8}{{23}} = \frac{{2 + 8}}{{23}} = \frac{{10}}{{23}}\]   Đ

b) \[\frac{5}{7} + \frac{4}{7} = \frac{{5 + 4}}{{7 + 7}} = \frac{9}{{14}}\]      S

c) \[\frac{3}{5} + \frac{1}{5} = \frac{{3 \times 1}}{{5 + 5}} = \frac{3}{{10}}\]    S

d) \[\frac{7}{9} + \frac{1}{9} = \frac{{7 \times 1}}{9} = \frac{7}{9}\]   S

Giải thích

b) \[\frac{5}{7} + \frac{4}{7} = \frac{{5 + 4}}{7} = \frac{9}{7}\]

c) \[\frac{3}{5} + \frac{1}{5} = \frac{{3 + 1}}{5} = \frac{4}{5}\]

d) \[\frac{7}{9} + \frac{1}{9} = \frac{{7 + 1}}{9} = \frac{8}{9}\]