cho phan thuc a =1/x+5+2/x-5 -2x+10/(x+5) (x-5) a) tim dieu kiem cua x de a co nghia

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1. Đáp án:

   $\begin{array}{l}
   A = \frac{1}{{x + 5}} + \frac{2}{{x – 5}} – \frac{{2x + 10}}{{\left( {x + 5} \right)\left( {x – 5} \right)}}\\
   a)Dkxd:\left\{ \begin{array}{l}
   x + 5 \ne 0\\
   x – 5 \ne 0
   \end{array} \right. \Rightarrow x \ne 5;x \ne  – 5\\
   b)A = \frac{1}{{x + 5}} + \frac{2}{{x – 5}} – \frac{{2x + 10}}{{\left( {x + 5} \right)\left( {x – 5} \right)}}\\
    = \frac{{x – 5 + 2\left( {x + 5} \right) – 2x – 10}}{{\left( {x + 5} \right)\left( {x – 5} \right)}}\\
    = \frac{{x – 5 + 2x + 10 – 2x – 10}}{{\left( {x + 5} \right)\left( {x – 5} \right)}}\\
    = \frac{{x – 5}}{{\left( {x + 5} \right)\left( {x – 5} \right)}}\\
    = \frac{1}{{x + 5}}
   \end{array}$

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