Bài 5: Thực hiện phép tính:
a) (-0,25).\(\dfrac{4}{17}\).\((-3\dfrac{5}{21})\).\((\dfrac{-7}{23})\); b) \((\dfrac{-2}{5})\).\(\dfrac{4}{15}\)+\((\dfrac{-3}{10})\).\(\dfrac{4}{15}\);
c) 21-3\(\dfrac{3}{4}\):\((\dfrac{3}{8}-\dfrac{1}{6})\); d) \((\dfrac{-5}{6}+\dfrac{2}{5})\):\(\dfrac{3}{8}\)+\((\dfrac{4}{5}-\dfrac{11}{30})\):\(\dfrac{3}{8}\)
\(a.\left(-0,25\right)\cdot\dfrac{4}{17}\cdot\left(-3\dfrac{5}{21}\right)\cdot\left(\dfrac{-7}{23}\right)\\ =\dfrac{-1}{4}\cdot\dfrac{4}{17}\cdot\dfrac{-68}{21}\cdot\dfrac{-7}{23}\\ =\dfrac{-1}{17}\cdot\dfrac{-68}{3}\cdot\dfrac{-1}{23}\\ =\dfrac{4}{3}\cdot\dfrac{-1}{23}\\ =-\dfrac{4}{69}\\ b.\left(-\dfrac{2}{5}\right)\cdot\dfrac{4}{15}+\left(\dfrac{-3}{10}\right)\cdot\dfrac{4}{15}\\ =\dfrac{4}{15}\cdot\left(-\dfrac{2}{5}+\dfrac{-3}{10}\right)\\ =\dfrac{4}{15}\cdot\dfrac{-7}{10}\\ =\dfrac{-14}{75}\\ c.21-3\dfrac{3}{4}:\left(\dfrac{3}{8}-\dfrac{1}{6}\right)\\= 21-\dfrac{15}{4}:\left(\dfrac{9}{24}-\dfrac{4}{24}\right)\\ =21-\dfrac{15}{4}\cdot\dfrac{24}{5}\\ =21-3\cdot6\\ =21-18\\=3\\ d.\left(-\dfrac{5}{6}+\dfrac{2}{5}\right):\dfrac{3}{8}+\left(\dfrac{4}{5}-\dfrac{11}{30}\right):\dfrac{3}{8}\\ =\left(-\dfrac{5}{6}+\dfrac{2}{5}\right)\cdot\dfrac{8}{3}+\left(\dfrac{4}{5}-\dfrac{11}{30}\right)\cdot\dfrac{8}{3}\\ =\dfrac{8}{3}\cdot\left(-\dfrac{5}{6}+\dfrac{2}{5}+\dfrac{4}{5}-\dfrac{11}{30}\right)\\ =\dfrac{8}{3}\left(\dfrac{6}{5}-\dfrac{6}{5}\right)\\ =0\)
