Thực hiện phép tính (tính hợp lí nếu có thể): a) 0,25 - 1,25.9;   b) 2/3 + (5/7+ - 2/3)

a) \(0,25 - 1,25.9\)

\( = 0,25 - 11,25\)

\( =  - 11\).

c) \(0,25.\frac{7}{{15}} - 25\% .\frac{{ - 8}}{{15}} + 2,75\)

\[ = 0,25.\frac{7}{{15}} + 0,25.\frac{8}{{15}} + 2,75\]

\[ = 0,25.\left( {\frac{7}{{15}} + \frac{8}{{15}}} \right) + 2,75\]

\[ = 0,25.\frac{{15}}{{15}} + 2,75\]

\[ = 0,25 + 2,75 = 3\].

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

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

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

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

d) \(\frac{5}{{17}}.\frac{{ - 7}}{3} + \frac{8}{{17}}.\frac{{ - 7}}{3} + \frac{{ - 7}}{3}.\frac{4}{{17}}\)

\[ = \frac{{ - 7}}{3}.\left( {\frac{5}{{17}} + \frac{8}{{17}} + \frac{4}{{17}}} \right)\]

\[ = \frac{{ - 7}}{3}.1 =  - \frac{7}{3}\].