Thực hiện phép tính (tính hợp lí nếu có thể): a) 1,25.6.(- 8);  b) 2/3 + (5/7 + - 2/3)

a) \(1,25.6.\left( { - 8} \right)\)

\( = \left[ {1,25.\left( { - 8} \right)} \right].6\)

\( = \left( { - 10} \right).6\)

\( =  - 60\).

b) \(\frac{2}{3} + \left( {\frac{5}{7} + \frac{{ - 2}}{3}} \right)\)

\( = \frac{2}{3} + \frac{5}{7} + \frac{{ - 2}}{3}\)

\[ = \left( {\frac{2}{3} + \frac{{ - 2}}{3}} \right) + \frac{5}{7}\]

\( = 0 + \frac{5}{7} = \frac{5}{7}\).

c) \(3,4.\left( { - 23,68} \right) - 3,4.45,12 + \left( { - 31,2} \right).3,4\)

\[ = 3,4.\left[ {\left( { - 23,68} \right) - 45,12 + \left( { - 31,2} \right)} \right]\]

\[ = 3,4.\left[ { - 100} \right]\]

\[ =  - 340\].

d) \(\frac{2}{7} + \frac{5}{7}.\left( {60\%  - 0,25} \right).{\left( { - 2} \right)^2}\)

\( = \frac{2}{7} + \frac{5}{7}.\left( {\frac{{60}}{{100}} - \frac{{25}}{{100}}} \right).4\)

\( = \frac{2}{7} + \frac{5}{7}.4.\frac{{35}}{{100}}\)

\( = \frac{2}{7} + \frac{{20}}{7}.\frac{7}{{20}}\)

\[ = \frac{2}{7} + 1\]\[ = \frac{9}{7}\].