B. TỰ LUẬN (3,0 điểm)

Câu 25-26. (1,5 điểm)

1) Thực hiện phép tính (tính hợp lí nếu có thể):

a) \(\frac{1}{4} \cdot \frac{7}{{15}} - \frac{1}{4} \cdot \frac{{ - 8}}{{15}} + 2\frac{3}{4}.\)                                  b) \[\frac{{ - 1}}{4} \cdot \left( {12\frac{3}{4} - 7,75} \right) - 25\% \cdot 3\frac{1}{2}.\]

1) a) \(\frac{1}{4} \cdot \frac{7}{{15}} - \frac{1}{4} \cdot \frac{{ - 8}}{{15}} + 2\frac{3}{4}\)

\( = \frac{1}{4} \cdot \left( {\frac{7}{{15}} - \frac{{ - 8}}{{15}}} \right) + \frac{{11}}{4}.\)

\( = \frac{1}{4} \cdot \frac{{15}}{{15}} + \frac{{11}}{4}\)

\( = \frac{1}{4} + \frac{{11}}{4}\)

\( = \frac{{12}}{4}\)

\( = 3.\)

b) \[\frac{{ - 1}}{4} \cdot \left( {12\frac{3}{4} - 7,75} \right) - 25\% \cdot 3\frac{1}{2}\]

\[ = \frac{{ - 1}}{4} \cdot \left( {12,75 - 7,75} \right) - \frac{{25}}{{100}} \cdot \frac{7}{2}\]

\[ = \frac{{ - 1}}{4} \cdot 5 - \frac{1}{4} \cdot \frac{7}{2}\]

\[ = \frac{{ - 5}}{4} - \frac{7}{8}\]

\[ = \frac{{ - 10}}{8} - \frac{7}{8}\]

\[ = \frac{{ - 17}}{8}.\]