Trong các phép tính dưới đây, phép tính có kết quả bé nhất là:

3/11

\[\frac{5}{7}\,\, - \,\,\frac{1}{3}\]

\[\frac{7}{{45}}\,\, \times \,\,\frac{9}{7}\]

\[\frac{7}{9}\,\,:\,\,\frac{2}{3}\]

\[\frac{4}{5}\,\, + \,\,\frac{9}{6}\]

Giải thích

Đáp án đúng là: B

\[\frac{5}{7}\,\, - \,\,\frac{1}{3}\,\, = \,\,\frac{{15}}{{21}}\,\, - \,\,\frac{7}{{21}}\,\, = \,\,\frac{8}{{21}}\]

\(\frac{7}{{45}}\,\, \times \,\,\frac{9}{7}\,\, = \,\,\frac{1}{5}\)
\[\frac{7}{9}\,\,:\,\,\frac{2}{3}\,\, = \,\,\frac{7}{9}\,\, & \times \,\,\frac{3}{2}\,\, = \,\,\frac{7}{6}\]

\[\frac{4}{5}\,\, + \,\,\frac{9}{6}\,\, = \,\,\frac{{24}}{{30}}\,\, + \,\,\frac{{45}}{{30}}\,\, = \,\,\frac{{69}}{{30}}\,\, = \,\,\frac{{23}}{{10}}\]

Ta có: \(\frac{8}{{21}}\,\, < \,\,1\); \(\frac{1}{5}\,\, < \,\,1\); \(\frac{7}{6}\,\, > \,\,1\); \(\frac{{23}}{{10}}\,\, > \,\,1\)

So sánh: \(\frac{8}{{21}}\) và \(\frac{1}{5}\)

\(\frac{8}{{21}}\,\, = \,\,\frac{{8\,\, \times \,\,5}}{{21\,\, \times \,\,5}}\,\, = \,\,\frac{{40}}{{105}}\)

\(\frac{1}{5}\,\, = \,\,\frac{{1\,\, \times \,\,21}}{{5\,\, \times \,\,21}}\,\, = \,\,\frac{{21}}{{105}}\)

Do đó: \(\frac{{21}}{{105}}\,\, < \,\,\frac{{40}}{{105}}\)

Phép tính có kết quả bé nhất là: \[\frac{7}{{45}}\,\, \times \,\,\frac{9}{7}\]