Giải sgk toán 6 tập 2: bài tập 80 trang 40

a) \(5.{{ - 3} \over {10}}=\frac{-3.5}{10}=\frac{-3}{2}\)

b) \({2 \over 7} + {5 \over 7}.{{14} \over {25}}=\frac{2}{7}+\frac{5.14}{7.25}=\frac{2}{7}+\frac{2}{5}=\frac{10}{35}+\frac{14}{35}=\frac{24}{35}\)

c) \({1 \over 3} - {5 \over 4}.{4 \over {15}}=\frac{1}{3}-\frac{1}{3}=0\)

d) \(\left( {{3 \over 4} + {{ - 7} \over 2}} \right).\left( {{2 \over {11}} + {{12} \over {22}}} \right)\)

\(=\left ( \frac{3}{4}-\frac{14}{4} \right ).\left ( \frac{4}{22}+\frac{12}{22} \right )\)

\(=\frac{-11}{4}.\frac{16}{22}=\frac{-11}{4}.\frac{8}{11}=\frac{-8}{4}=-2\)