Tính bằng cách thuận tiện nhất 5/ 13 × 11/ 19 + 11/ 19 × 21/ 13 = .

\[\frac{5}{{13}} \times \frac{{11}}{{19}} + \frac{{11}}{{19}} \times \frac{{21}}{{13}} = \frac{{11}}{{19}} \times \left( {\frac{5}{{13}} + \frac{{21}}{{13}}} \right) = \frac{{11}}{{19}} \times \frac{{26}}{{13}} = \frac{{11 \times 2 \times \cancel{{13}}}}{{19 \times \cancel{{13}}}} = \frac{{22}}{{19}}\]