Giải thích các bước giải:
\[\begin{array}{l}
a,\frac{{{x^2} - 4}}{{{x^2} - 1}}\\
\Rightarrow {x^2} - 1 \ne 0 \Leftrightarrow \left( {x - 1} \right)\left( {x + 1} \right) \ne 0 \Rightarrow \left\{ \begin{array}{l}
x \ne 1\\
x \ne - 1
\end{array} \right.\\
b,\\
\frac{2}{{\left( {x + 1} \right)\left( {x - 3} \right)}}\\
\Rightarrow \left( {x + 1} \right)\left( {x - 3} \right) \ne 0 \Rightarrow \left\{ \begin{array}{l}
x \ne - 1\\
x \ne 3
\end{array} \right.\\
c,\\
\frac{{2x + 1}}{{{x^2} - 5x + 6}}\\
\Rightarrow {x^2} - 5x + 6 \ne 0 \Leftrightarrow \left( {x - 2} \right)\left( {x - 3} \right) \ne 0\\
\Leftrightarrow \left\{ \begin{array}{l}
x \ne 2\\
x \ne 3
\end{array} \right.\\
d,\\
\frac{{2x + 3}}{{4x - 5}} \Rightarrow 4x - 5 \ne 0 \Leftrightarrow x \ne \frac{5}{4}\\
e,\\
\frac{{{x^2} - 1}}{{{x^2} - 2x + 1}} \to {x^2} - 2x + 1 \ne 0 \Leftrightarrow {\left( {x - 1} \right)^2} \ne 0 \Leftrightarrow x \ne 1
\end{array}\]
