Đáp án: 49/156
Giải thích các bước giải:
$\begin{array}{l}
\frac{7}{{13}}.\frac{7}{{15}} - \frac{5}{{12}}.\frac{{21}}{{39}} + \frac{{49}}{{91}}.\frac{8}{{15}}\\
= \frac{7}{{13}}.\frac{7}{{15}} - \frac{5}{{12}}.\frac{7}{{13}} + \frac{7}{{13}}.\frac{8}{{15}}\\
= \frac{7}{{13}}\left( {\frac{7}{{15}} - \frac{5}{{12}} + \frac{8}{{15}}} \right)\\
= \frac{7}{{13}}.\left( {1 - \frac{5}{{12}}} \right)\\
= \frac{7}{{13}}.\frac{7}{{12}}\\
= \frac{{49}}{{156}}
\end{array}$