Thực hiện phép tính (hợp lý nếu được) (a) − 3/7 + − 2/3 . (b) − 13 , 25 + 31 , 58 .

a) \(\frac{{ - 3}}{7} + \frac{{ - 2}}{3}\)

\( = \frac{{ - 9}}{{21}} + \frac{{ - 14}}{{21}}\)

\( = \frac{{ - 23}}{{21}}\).

b) \( - 13,25 + 31,58\)

\( = 31,58 - 13,25\)

\( = 18,33\).

c) \(\frac{5}{{11}}.\frac{{ - 3}}{8} - \frac{5}{{11}}.\frac{5}{8} + 2\frac{5}{{11}}\)

\( = \frac{5}{{11}}.\frac{{ - 3}}{8} - \frac{5}{{11}}.\frac{5}{8} + \frac{{27}}{{11}}\)

\( = \frac{5}{{11}}.\left( {\frac{{ - 3}}{8} - \frac{5}{8}} \right) + \frac{{27}}{{11}}\)

\( = \frac{5}{{11}}.( - 1) + \frac{{27}}{{11}}\)

\( = \frac{{ - 5}}{{11}} + \frac{{27}}{{11}}\)

\( = \frac{{22}}{{11}} = 2\).

d) \(\frac{2}{{13}} - \left( {\frac{{15}}{{17}} + \frac{2}{{13}}} \right)\)

\( = \frac{2}{{13}} - \frac{{15}}{{17}} - \frac{2}{{13}}\)

\( = \left( {\frac{2}{{13}} - \frac{2}{{13}}} \right) - \frac{{15}}{{17}}\)

\( = 0 - \frac{{15}}{{17}} = - \frac{{15}}{{17}}\) .