Dãy số nào dưới đây được sắp xếp theo thứ tự từ lớn đến bé?  

a)

\(\frac{2}{5} + \frac{1}{6} = \frac{{12}}{{30}} + \frac{5}{{30}} = \frac{7}{{30}}\)

So sánh: \(\frac{7}{{30}} < \frac{{23}}{{30}}\)

Vậy: \(\frac{2}{5} + \frac{1}{6}\,\, < \frac{{23}}{{30}}\)   

b)

\(\frac{2}{3} - \frac{5}{8} = \frac{{16}}{{24}} - \frac{{15}}{{24}} = \frac{1}{{24}}\)

So sánh: \(\frac{1}{{24}} < \frac{1}{2}\)

Vậy: \(\frac{2}{3} - \frac{5}{8}\,\, < \,\frac{1}{2}\)

  c)

\(\frac{7}{5} \times \frac{3}{4} = \frac{{21}}{{20}}\)

\(\frac{3}{5}:\frac{1}{9} = \frac{3}{5} \times 9 = \frac{{27}}{5} = \frac{{108}}{{20}}\)

So sánh: \(\frac{{21}}{{20}} < \frac{{108}}{{20}}\)

Vậy: \(\frac{7}{5} \times \frac{3}{4}\,\, < \frac{3}{5}:\frac{1}{9}\)

d)

\(\frac{2}{3} + \frac{1}{5} = \frac{{10}}{{15}} + \frac{3}{{15}} = \frac{{13}}{{15}} = \frac{{26}}{{30}}\)

\(\frac{9}{5}:\frac{6}{7} = \frac{9}{5} \times \frac{7}{6} = \frac{{63}}{{30}}\)

So sánh: \(\frac{{26}}{{30}} < \frac{{63}}{{30}}\)

Vậy: \(\frac{2}{3} + \frac{1}{5}\,\, < \,\frac{9}{5}:\frac{6}{7}\)