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

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

    

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