Tính giá trị biểu thức:(a) (3/5 + -2/7) + -1/5

a) \[\left( {\frac{3}{5} + \frac{{ - 2}}{7}} \right) + \frac{{ - 1}}{5} = \left( {\frac{{21}}{{35}} + \frac{{ - 10}}{{35}}} \right) + \frac{{ - 1}}{5} = \frac{{11}}{{35}} + \frac{{ - 7}}{5} = \frac{4}{{35}}\].

b) \[ - \left( {\frac{{ - 3}}{4}} \right) - \left( {\frac{2}{3} + \frac{1}{4}} \right) = \frac{3}{4} - \left( {\frac{8}{{12}} + \frac{3}{{12}}} \right) = \frac{9}{{12}} - \frac{{11}}{{12}} = \frac{{ - 2}}{{12}} = \frac{{ - 1}}{6}\].