1)Tìm x, biết : a)2,5:13,5=x:3/5 b) -6/x=4/7/1+1/3 d)-3/x-1=8/15

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 $x=-\dfrac{37}{8}$