Thực hiện phép tính (tính nhanh nếu có thể) a) 1/4 − 3/8 : ( − 3/5 ) ; b) 2 /− 5 + − 3/10 + 1/5 + 1/2 ;

a) \(\frac{1}{4} - \frac{3}{8}:\left( {\frac{{ - 3}}{5}} \right)\)\( = \frac{1}{4} - \frac{{3\,.\,5}}{{8\,.\,\left( { - 3} \right)}} = \frac{1}{4} - \frac{{ - 5}}{8}\)\( = \frac{1}{4} + \frac{5}{8} = \frac{2}{8} + \frac{5}{8} = \frac{7}{8}\).

b) \[\frac{2}{{ - 5}} + \frac{{ - 3}}{{10}} + \frac{1}{5} + \frac{1}{2}\]\[ = \frac{{ - 2}}{5} + \frac{{ - 3}}{{10}} + \frac{1}{5} + \frac{1}{2} = \left( {\frac{{ - 2}}{5} + \frac{1}{5}} \right) + \left( {\frac{{ - 3}}{{10}} + \frac{1}{2}} \right)\]

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

c) \(\left( { - 25,2} \right):\left[ {1\frac{1}{5} \cdot 3 - \frac{1}{{15}} - \frac{8}{9} \cdot \left( { - 0,75} \right)} \right]\)

\( = \left( { - 25,2} \right):\left( {\frac{6}{5} \cdot 3 - \frac{1}{{15}} + \frac{2}{3}} \right)\)\( = \left( { - 25,2} \right):\left( {\frac{{18}}{5} - \frac{1}{{15}} + \frac{2}{3}} \right)\)

\( = \left( { - 25,2} \right):\left( {\frac{{54}}{{15}} - \frac{1}{{15}} + \frac{{10}}{{15}}} \right)\)\( = \left( { - 25,2} \right):\frac{{21}}{5}\)\( = \frac{{ - 126}}{{21}} = - 6.\)

d) \({\left( { - \frac{3}{2}} \right)^2} \cdot 4 + \sqrt {1\frac{5}{4}} :2\frac{1}{2} - \left| {\frac{{ - 3}}{4}} \right|\)

\( = \frac{9}{4} \cdot 4 + \sqrt {\frac{9}{4}} :\frac{5}{2} - \frac{3}{4}\)\( = 9 + \frac{3}{2}:\frac{5}{2} - \frac{3}{4}\)

\( = 9 + \frac{3}{5} - \frac{3}{4} = 9 + \left( {\frac{3}{5} - \frac{3}{4}} \right)\)\( = 9 - \frac{3}{{20}}\)\( = \frac{{177}}{{20}}\).