Tìm x, biết: a)2/5:x=-1/4 b)hỗn số 1 3/4x-5= hỗn số 3 1/3 c) 2x-3/7= hỗn số 6 2/7 d) 2/3x+1/4=7/12 e) Hỗn số 3 1/3+5/6x= hỗn số 3 1/2 f) 2/3+1/3:x=3/

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1. Đáp án:

   $\begin{array}{l}
   a)\frac{2}{5}:x =  – \frac{1}{4}\\
    \Rightarrow x = \frac{2}{5}:\left( { – \frac{1}{4}} \right)\\
    \Rightarrow x = \frac{2}{5}.\left( { – 4} \right) = \frac{{ – 8}}{5}\\
   b)1\frac{3}{4}.x – 5 = 3\frac{1}{3}\\
    \Rightarrow \frac{7}{4}x – 5 = \frac{{10}}{3}\\
    \Rightarrow \frac{7}{4}x = \frac{{10}}{3} + 5 = \frac{{25}}{3}\\
    \Rightarrow x = \frac{{25}}{3}:\frac{7}{4}\\
    \Rightarrow x = \frac{{25}}{3}.\frac{4}{7}\\
    \Rightarrow x = \frac{{100}}{{21}}\\
   c)2x – \frac{3}{7} = 6\frac{2}{7}\\
    \Rightarrow 2x – \frac{3}{7} = \frac{{44}}{7}\\
    \Rightarrow 2x = \frac{{44}}{7} + \frac{3}{7} = \frac{{47}}{7}\\
    \Rightarrow x = \frac{{47}}{{14}}\\
   d)\frac{2}{3}x + \frac{1}{4} = \frac{7}{{12}}\\
    \Rightarrow \frac{2}{3}x = \frac{7}{{12}} – \frac{1}{4} = \frac{{7 – 3}}{{12}} = \frac{1}{3}\\
    \Rightarrow x = \frac{1}{3}:\frac{2}{3} = \frac{1}{2}\\
   e)3\frac{1}{3} + \frac{5}{6}x = 3\frac{1}{2}\\
    \Rightarrow \frac{{10}}{3} + \frac{5}{6}x = \frac{7}{2}\\
    \Rightarrow \frac{5}{6}x = \frac{{10}}{3} – \frac{7}{2} = \frac{{ – 1}}{6}\\
    \Rightarrow x =  – \frac{1}{5}\\
   f)\frac{2}{3} + \frac{1}{3}:x = \frac{3}{5}\\
    \Rightarrow \frac{1}{3}:x = \frac{3}{5} – \frac{2}{3} = \frac{{ – 1}}{{15}}\\
    \Rightarrow x = \frac{1}{3}:\left( { – \frac{1}{{15}}} \right)\\
    \Rightarrow x =  – 5
   \end{array}$

   Vậy x=-5

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