Tính giá trị của các biểu thức sau (tính hợp lí nếu có thể): a) 7/ 4 − 3/ 4 : 12 /21 ; b) √ 4/ 9 − ∣ ∣ − 3/ 7 ∣ ∣ . 7/ 8 ;

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) \[\sqrt {\frac{4}{9}} - \left| {\frac{{ - 3}}{7}} \right|.\frac{7}{8} = \frac{2}{3} - \frac{3}{7}.\frac{7}{8} = \frac{2}{3} - \frac{3}{8} = \frac{{16}}{{24}} - \frac{9}{{24}} = \frac{5}{{24}}\].

c) \(\left( {\frac{1}{3} - \frac{3}{{10}}} \right):\frac{3}{5} + \left( {\frac{2}{3} - \frac{7}{{10}}} \right):\frac{3}{5}\)\( = \left( {\frac{1}{3} - \frac{3}{{10}}} \right).\frac{5}{3} + \left( {\frac{2}{3} - \frac{7}{{10}}} \right).\frac{5}{3}\)

\( = \left( {\frac{1}{3} - \frac{3}{{10}} + \frac{2}{3} - \frac{7}{{10}}} \right).\frac{5}{3} = \left( {1 - 1} \right).\frac{5}{3} = 0\).