Giải thích các bước giải:
a, $\dfrac{5}{12}$ - $\dfrac{21}{8}$ × $\dfrac{1}{14}$
= $\dfrac{5}{12}$ - $\dfrac{3}{16}$
= $\dfrac{20}{48}$ - $\dfrac{9}{48}$
= $\dfrac{11}{48}$
b, $\dfrac{3}{4}$ + $\dfrac{1}{4}$ × $\dfrac{8}{5}$
= $\dfrac{3}{4}$ - $\dfrac{2}{5}$
= $\dfrac{15}{20}$ - $\dfrac{8}{20}$
= $\dfrac{7}{20}$
c, ($\dfrac{19}{21}$ - $\dfrac{2}{3}$) × 2,8
= ($\dfrac{19}{21}$ - $\dfrac{14}{21}$) × 2,8
= $\dfrac{5}{21}$ × 2,8
= $\dfrac{2}{3}$
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