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

Đáp án:

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