a: \(\dfrac{7}{12}\cdot3\dfrac{2}{5}-\dfrac{7}{12}\cdot9\dfrac{2}{5}\)
\(=\dfrac{7}{12}\left(3+\dfrac{2}{5}-9-\dfrac{2}{5}\right)\)
\(=\dfrac{7}{12}\cdot\left(-6\right)=-\dfrac{7}{2}\)
b: \(2\dfrac{6}{31}-\dfrac{5}{24}+\dfrac{25}{31}-1\dfrac{1}{24}+0,25\)
\(=\left(2+\dfrac{6}{31}+\dfrac{25}{31}\right)-\left(\dfrac{5}{24}+1+\dfrac{1}{24}\right)+0,25\)
\(=\left(2+1\right)-\left(\dfrac{1}{4}+1\right)+0,25\)
\(=3-\dfrac{5}{4}+\dfrac{1}{4}=3-1=2\)
c: \(1\dfrac{8}{15}:\left(-\dfrac{3}{5}\right)-5\dfrac{11}{15}:\left(-\dfrac{3}{5}\right)\)
\(=\left(1+\dfrac{8}{15}-5-\dfrac{11}{15}\right):\dfrac{-3}{5}\)
\(=\left(-4-\dfrac{3}{15}\right):\dfrac{-3}{5}=\left(4+\dfrac{1}{5}\right):\dfrac{3}{5}=\dfrac{21}{5}\cdot\dfrac{5}{3}=\dfrac{21}{3}\)
=7
c) \(1\dfrac{8}{15}:\left(-\dfrac{3}{5}\right)-5\dfrac{11}{15}:\left(-\dfrac{3}{5}\right)\)
= \(\dfrac{23}{15}.\dfrac{-5}{3}-\dfrac{86}{15}.\dfrac{-5}{3}\)
= \(\dfrac{-5}{3}.\left(\dfrac{23}{15}-\dfrac{86}{15}\right)\)
=\(\dfrac{-5}{3}.\dfrac{-63}{15}\)
= \(\dfrac{-5}{3}.\dfrac{-21}{5}\)
= \(\dfrac{\left(-1\right).\left(-7\right)}{1.1}\)
=7
