Thực hiện phép tính (tính hợp lí nếu có thể): (a) 7/ 4 − 3/ 4 : 12/ 21

a) \(\frac{7}{4} - \frac{3}{4}:\frac{{12}}{{21}}\)\( = \frac{7}{4} - \frac{3}{4}.\frac{{21}}{{12}}\)\( = \frac{7}{4} - \frac{{21}}{{16}}\)\( = \frac{7}{{16}}\).

b) \(\frac{{ - 9}}{8}.{\left( {\frac{2}{3}} \right)^3} + \frac{{ - 4}}{9}\)\( = \frac{{ - 9}}{8}.\frac{8}{{27}} + \frac{{ - 4}}{9}\)\( = \frac{{ - 1}}{3} + \frac{{ - 4}}{9}\)\( = - \frac{7}{9}\).

c) \(\frac{1}{6}.\frac{4}{9} - \frac{{13}}{9}.\frac{1}{6} + \frac{1}{6}\)\( = \frac{1}{6}.\left( {\frac{4}{9} - \frac{{13}}{9} + 1} \right)\)\( = \frac{1}{6}.0 = 0\).