tính hợp lí :

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1. Đáp án:

   $\begin{array}{l}
   a)\dfrac{{13}}{{42}} + \dfrac{{ – 5}}{{18}} – \dfrac{{11}}{{21}} – \dfrac{{ – 4}}{{14}}\\
    = \dfrac{{13}}{{42}} – \dfrac{{22}}{{42}} + \dfrac{{4.3}}{{42}} – \dfrac{5}{{18}}\\
    = \dfrac{{13 – 22 + 12}}{{42}} – \dfrac{5}{{18}}\\
    = \dfrac{3}{{42}} – \dfrac{5}{{18}}\\
    = \dfrac{1}{{14}} – \dfrac{5}{{18}}\\
    = \dfrac{{9 – 35}}{{126}}\\
    = \dfrac{{ – 26}}{{126}} = \dfrac{{ – 13}}{{63}}\\
   b)\dfrac{{12}}{{35}} – \dfrac{{17}}{{14}} + \dfrac{{11}}{{10}} – \left( {\dfrac{{ – 5}}{{28}}} \right)\\
    = \dfrac{{12}}{{35}} + \dfrac{{11}}{{10}} – \dfrac{{34}}{{28}} + \dfrac{5}{{28}}\\
    = \dfrac{{12}}{{35}} + \dfrac{{11}}{{10}} – \dfrac{{29}}{{28}}\\
    = \dfrac{{12}}{{35}} + \dfrac{1}{{10}} + 1 – \left( {1 + \dfrac{1}{{28}}} \right)\\
    = \dfrac{{12}}{{35}} + \dfrac{1}{{10}} – \dfrac{1}{{28}}\\
    = \dfrac{{12.4 – 5}}{{35.4}} + \dfrac{1}{{10}}\\
    = \dfrac{{43}}{{140}} + \dfrac{1}{{10}}\\
    = \dfrac{{43 + 14}}{{140}}\\
    = \dfrac{{57}}{{140}}
   \end{array}$

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