Đáp án:
$\begin{array}{l}
a)\\
- \frac{1}{3} + \frac{4}{5} = \frac{{ - 5}}{{15}} + \frac{{4.3}}{{15}} = \frac{{ - 5 + 12}}{{15}} = \frac{7}{{15}}\\
b)\\
\left( {\frac{{ - 2}}{7} + \frac{5}{6}} \right):\frac{5}{{12}} + \left( { - \frac{5}{7} + \frac{1}{6}} \right):\frac{5}{{12}}\\
= \frac{{ - 2.6 + 5.7}}{{42}}.\frac{{12}}{5} + \frac{{ - 5.6 + 7}}{{42}}.\frac{{12}}{5}\\
= \frac{{23}}{{42}}.\frac{{12}}{5} + \frac{{ - 23}}{{42}}.\frac{{12}}{5}\\
= 0
\end{array}$
