Thực hiện phép tínha. \(\frac{1}{7}.\frac{{ - 3}}{8} + \frac{{ - 13}}{8}.\frac{1}{7}\)b. \(9.

\(a)\frac{1}{7}.\frac{{ - 3}}{8} + \frac{{ - 13}}{8}.\frac{1}{7} = \frac{1}{7}.\left( {\frac{{ - 3}}{8} + \frac{{ - 13}}{8}} \right) = \frac{1}{7}.\left( { - 2} \right) = \frac{{ - 2}}{7}\)

\(\begin{array}{l}
b)9.{\left( {\frac{1}{3}} \right)^2}:\left[ {{{\left( { - \frac{2}{3}} \right)}^2} + 0,5 - 1\frac{1}{2}} \right]\\
 = 9.\frac{1}{{27}}:\left( {\frac{4}{9} + \frac{1}{2} - \frac{3}{2}} \right) = \frac{1}{3}:\left( {\frac{4}{9} - \frac{2}{2}} \right)\\
 = \frac{1}{3}:\frac{{ - 5}}{9} = \frac{1}{3}.\frac{9}{{ - 5}} =  - \frac{3}{5}
\end{array}\)

\(c)\frac{{ - 5}}{9}.\left( {\frac{3}{{10}} - \frac{2}{5}} \right) = \frac{{ - 5}}{9}.\left( {\frac{3}{{10}} - \frac{4}{{10}}} \right) = \frac{{ - 5}}{9}.\frac{{ - 1}}{{10}} = \frac{1}{{18}}\)