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Bài làm

`a,` $\dfrac{-3}{7}$ `+` $\dfrac{5}{12}$ `÷` $\dfrac{- 5}{8}$

`= `$\dfrac{-3}{7}$ `+` $\dfrac{5}{12}$ `.`  $\dfrac{- 8}{5}$

`=` $\dfrac{-3}{7}$ `+` $\dfrac{-2}{3}$

`=` $\dfrac{-9-14}{21}$

`=` $\dfrac{-23}{21}$

`b,` $\dfrac{4}{9}$ `.` $\dfrac{7}{17}$ `- `$\dfrac{2}{9}$ `.` $\dfrac{4}{17}$ `+` $\dfrac{4}{9}$ `.` $\dfrac{12}{17}$

`=` $\dfrac{4}{9}$ `. (` $\dfrac{7}{17}$ `+` $\dfrac{12}{17}$ `) -` $\dfrac{2}{9}$ `.` $\dfrac{4}{17}$

`=` $\dfrac{4}{9}$ `.` $\dfrac{19}{17}$ `-` $\dfrac{2}{9}$ `.` $\dfrac{4}{17}$

`=` $\dfrac{76}{153}$ `-` $\dfrac{8}{153}$

`=` $\dfrac{68}{153}$ `=` $\dfrac{4}{9}$

`c, (` $\dfrac{1}{6}$ `+` $\dfrac{3}{4}$ `÷ ( - 3 )² - 50% ) ÷ [` $\dfrac{5}{7}$ `-` `(` $\dfrac{3}{8}$ `-` $\dfrac{2}{7}$ ) `+` $\dfrac{3}{8}$ `]`

`= (` $\dfrac{1}{6}$ `+` $\dfrac{3}{4}$ `×` $\dfrac{1}{9}$ `-` $\dfrac{1}{2}$ `) ÷ (` $\dfrac{5}{7}$ - $\dfrac{3}{8}$ `+` $\dfrac{2}{7}$ `+`  $\dfrac{3}{8}$ `)`

`= (` $\dfrac{1}{6}$ `+` $\dfrac{1}{12}$ - $\dfrac{3}{6}$ `) × 1`

`=` $\dfrac{-4}{12}$ `+` $\dfrac{1}{12}$

`=` $\dfrac{-1}{4}$

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