Tính rồi rút gọn:

a) \(\frac{1}{3} \times \frac{1}{5} = \frac{{1 \times 1}}{{3 \times 5}} = \frac{1}{{15}}\)

\(\frac{3}{5} \times \frac{2}{7} = \frac{{3 \times 2}}{{5 \times 7}} = \frac{6}{{35}}\)

b) \(\frac{1}{4} \times \frac{3}{4} = \frac{{1 \times 3}}{{4 \times 4}} = \frac{3}{{16}}\)

\(\frac{9}{{11}} \times \frac{5}{8} = \frac{{9 \times 5}}{{11 \times 8}} = \frac{{45}}{{88}}\)