Đáp án + Giải thích các bước giải:

a)  $\dfrac{4}{21}$ + $\dfrac{3}{5}$ + $\dfrac{17}{21}$ − $\dfrac{7}{5}$

 = ( $\dfrac{4}{21}$ + $\dfrac{17}{21}$ ) + ( $\dfrac{3}{5}$ + $\dfrac{−7}{5}$ )

 = 1 + $\dfrac{−4}{5}$

 = $\dfrac{5}{5}$ + $\dfrac{−4}{5}$

 = + $\dfrac{1}{5}$

b) $\dfrac{−7}{8}$ $\times$ $\dfrac{3}{9}$ - $\dfrac{5}{6}$ + $\dfrac{−7}{8}$ $\times$ $\dfrac{16}{19}$

= $\dfrac{−7}{8}$ $\times$ ( $\dfrac{3}{19}$ + $\dfrac{16}{19}$ ) - $\dfrac{5}{6}$

= $\dfrac{−7}{8}$ $\times$ 1 - $\dfrac{5}{6}$

= $\dfrac{−7}{8}$ − $\dfrac{5}{6}$

= $\dfrac{−21}{24}$ − $\dfrac{20}{24}$

= $\dfrac{−41}{24}$.

@namlebao

    Chúc cậu học giỏi nha, PLS câu trả lời hay nhất.