MSC: 21

\(\dfrac{3}{7}=\dfrac{9}{21}\) ; \(\dfrac{4}{3}=\dfrac{28}{21}\) ; \(\dfrac{8}{21}\)

\(2=\dfrac{90}{45}\)

\(n_{Al}=\dfrac{8,1}{27}=0,3\left(mol\right)\)

\(n_{HCl}=\dfrac{21,9}{36,5}=0,6\left(mol\right)\)

\(2Al+6HCl\xrightarrow[]{}2AlCl_3+3H_2\uparrow\)

 0,2  ←  0,6   →   0,2   →   0,3

a) \(m_{AlCl_3}=0,2\cdot133,5=26,7\left(g\right)\)

b) \(CuO+H_2\xrightarrow[]{t^\circ}Cu+H_2O\uparrow\)

      0,3 ← 0,3

\(m_{CuO}=0,3\cdot80=24\left(g\right)\)

\(\dfrac{2}{3}=\dfrac{2\times4}{3\times4}=\dfrac{8}{12}\) và \(\dfrac{5}{12}\)

MSC: 299

\(\dfrac{14}{13}=\dfrac{322}{299}\) ; \(\dfrac{4}{-23}=-\dfrac{52}{299}\)

a) \(-2+\left(-\dfrac{5}{8}\right)=-\dfrac{16}{8}+\left(-\dfrac{5}{8}\right)=-\dfrac{21}{8}\)

b) \(5+\left(-\dfrac{3}{4}\right)=\dfrac{20}{4}+\left(-\dfrac{3}{4}\right)=\dfrac{17}{4}\)

c) \(\dfrac{4}{3}+\left(-6\right)=\dfrac{4}{3}+\left(-\dfrac{18}{3}\right)=-\dfrac{14}{3}\)

d) \(-8-\dfrac{2}{7}=-\dfrac{56}{7}-\dfrac{2}{7}=-\dfrac{58}{7}\)

e) \(-\dfrac{3}{5}-9=-\dfrac{3}{5}-\dfrac{45}{5}=-\dfrac{48}{5}\)

g) \(6-\left(-\dfrac{4}{13}\right)=6+\dfrac{4}{13}=\dfrac{78}{13}+\dfrac{4}{13}=\dfrac{82}{13}\)

a) \(\dfrac{1}{8}-\dfrac{1}{2}=\dfrac{1}{8}-\dfrac{4}{8}=-\dfrac{3}{8}\)

b) \(-\dfrac{11}{12}-\left(-1\right)=-\dfrac{11}{12}+\dfrac{12}{12}=\dfrac{1}{12}\)

c) \(\dfrac{3}{5}-\dfrac{5}{6}=\dfrac{18}{30}-\dfrac{25}{30}=-\dfrac{7}{30}\)

d) \(-\dfrac{1}{16}-\dfrac{1}{15}=-\dfrac{15}{240}-\dfrac{16}{240}=-\dfrac{31}{240}\)

e) \(\dfrac{11}{36}-\left(-\dfrac{7}{24}\right)=\dfrac{11}{36}+\dfrac{7}{24}=\dfrac{22}{72}+\dfrac{21}{72}=\dfrac{43}{72}\)

g) \(-\dfrac{5}{9}-\left(-\dfrac{5}{12}\right)=-\dfrac{5}{9}+\dfrac{5}{12}=-\dfrac{20}{36}+\dfrac{15}{36}=-\dfrac{5}{36}\)

MSC: 120

\(-\dfrac{13}{30}=-\dfrac{52}{120}\); \(\dfrac{21}{-40}=-\dfrac{63}{120}\)

Sửa đáp số lại giúp mình nhé: \(-\dfrac{19}{46}\)

\(\left(-\dfrac{7}{23}\right)\cdot\left(\dfrac{13}{28}\right)+\left(-\dfrac{7}{13}\right)\cdot\left(\dfrac{25}{28}\right)\)

\(=\left(-\dfrac{7}{23}\right)\cdot\left(\dfrac{13}{28}+\dfrac{25}{28}\right)\)

\(=\left(-\dfrac{7}{13}\right)\cdot\dfrac{38}{28}\)

\(=\left(-\dfrac{7}{13}\right)\cdot\dfrac{19}{14}\)

\(=-\dfrac{19}{26}\)