Thực hiện phép tính (tính hợp lý nếu có thể): (a) − 2/9 − 5/9 + 4/9 . (b) 5/18 + − 7/12 .

a) \(\frac{{ - 2}}{9} - \frac{5}{9} + \frac{4}{9} = \frac{{ - 2 - 5 + 4}}{9} = \frac{{ - 3}}{9} = \frac{{ - 1}}{3}\).

b) \(\frac{5}{{18}} + \frac{{ - 7}}{{12}} = \frac{{10}}{{36}} + \frac{{ - 21}}{{36}} = \frac{{ - 11}}{{36}}\).

c) \(\frac{{ - 6}}{{21}} + \frac{{12}}{{ - 44}} + \frac{{ - 10}}{{14}} - \left( {\frac{{ - 18}}{{33}} - 1\frac{1}{2}} \right) = \frac{{ - 2}}{7} + \frac{{ - 3}}{{11}} + \frac{{ - 5}}{7} + \frac{6}{{11}} + \frac{3}{2}\)

\( = \left( {\frac{{ - 2}}{7} + \frac{{ - 5}}{7}} \right) + \left( {\frac{{ - 3}}{{11}} + \frac{6}{{11}}} \right) + \frac{3}{2}\)

\( = - 1 + \frac{3}{{11}} + \frac{3}{2} = \frac{{ - 22 + 6 + 33}}{{22}} = \frac{{17}}{{22}}\).