Giải bài 98 trang 49 - SGK Toán lớp 7 Tập 1

a) $-\dfrac{3}{5}.y = \dfrac{21}{10}$

$\Leftrightarrow y = \dfrac{21}{10} : \left(-\dfrac{3}{5}\right)$

$\Leftrightarrow y = \dfrac{21}{10} . \left(-\dfrac{5}{3}\right)$

$\Leftrightarrow y = -\dfrac{7}{2} = -3\dfrac{1}{2}$

Vậy $y = -3\dfrac{1}{2}$

b) $y : \dfrac{3}{8} = -1\dfrac{31}{33}$

$\Leftrightarrow y : \dfrac{3}{8} = -\dfrac{64}{33}$

$\Leftrightarrow y = -\dfrac{64}{33} .\dfrac{3}{8}$

$\Leftrightarrow y = -\dfrac{8}{11}$

Vậy $y = -\dfrac{8}{11}$

c) $1\dfrac{2}{5}.y + \dfrac{3}{7} = -\dfrac{4}{5}$

$\Leftrightarrow \dfrac{7}{5}.y + \dfrac{3}{7} = -\dfrac{4}{5}$

$\Leftrightarrow \dfrac{7}{5}.y = -\dfrac{4}{5} - \dfrac{3}{7}$

$\Leftrightarrow \dfrac{7}{5}.y = \dfrac{-28 - 15}{35}$

$\Leftrightarrow \dfrac{7}{5}.y = \dfrac{-43}{35}$

$\Leftrightarrow y = \dfrac{-43}{35} : \dfrac{7}{5}$

$\Leftrightarrow y = \dfrac{-43}{35} .\dfrac{5}{7} = \dfrac{-43}{49}$

d) $-\dfrac{11}{12} . y + 0,25 = \dfrac{5}{6}$

$\Leftrightarrow -\dfrac{11}{12} . y = \dfrac{5}{6} - 0,25$

$\Leftrightarrow -\dfrac{11}{12} . y = \dfrac{10}{12} - \dfrac{3}{12}$

$\Leftrightarrow -\dfrac{11}{12} . y = \dfrac{7}{12}$

$\Leftrightarrow y = \dfrac{7}{12} : \left(-\dfrac{11}{12}\right)$

$\Leftrightarrow y = \dfrac{7}{12} . \left(-\dfrac{12}{11}\right) = - \dfrac{7}{11}$

Vậy $y = -\dfrac{7}{11}$

Lưu ý:    $\dfrac{a}{b} : \dfrac{c}{d} = \dfrac{a}{b} . \dfrac{d}{c}$