Đáp án:
$a) 2,5 : 13,5 = x : \dfrac{3}{5}$
$⇔\dfrac{5}{27} = x : \dfrac{3}{5}$
$⇔\dfrac{5}{27} . \dfrac{3}{5} = x$
$⇔\dfrac{1}{9} = x$
Vậy $x=\dfrac{1}{9}$
$b) \dfrac{-6}{x} =\dfrac{4}{7} + \dfrac{1}{3} $
$⇔\dfrac{-6}{x} = \dfrac{19}{21}$
$⇔ x = \dfrac{-6 . 21}{19}$
$⇔x = -\dfrac{38}{7}$
Vậy $x=-\dfrac{38}{7}$
$d) \dfrac{-3}{x-1}= \dfrac{8}{15}$
$\text{ĐKXĐ : x $\neq$ 1}$
$⇔\dfrac{-3.15}{15(x-1)} = \dfrac{8(x-1)}{15(x-1)}$
$⇔\dfrac{-45}{15(x-1)} = \dfrac{8(x-1)}{15(x-1)}$
$⇔ -45 = 8(x-1)$
$⇔ -45 = 8x -8$
$⇔ -45 +8 = 8x$
$⇔-37 = 8x$
$⇔-\dfrac{37}{8} =x$ (thỏa mãn)
Vậy $x=-\dfrac{37}{8}$
