a) $\left\{ \begin{array}{l}2-x≥0\\x-3≥0\end{array} \right.$
$↔ \left\{ \begin{array}{l}x≤2\\x≥3\end{array} \right.$
$→ x∈∅$
TXĐ là: $D=∅$
b) $\left\{ \begin{array}{l}3x≥0\\-2x≥0\end{array} \right.$
$↔ \left\{ \begin{array}{l}x≥0\\x≤0\end{array} \right.$
$↔ x=0$
TXĐ là: $D=\{0\}$
c) TXĐ là: $D=R$
d) $\left\{ \begin{array}{l}x+1>0\\x+4≥0\\x^2-4x+3 \neq 0\end{array} \right.$
$↔ \left\{ \begin{array}{l}x>-1\\\left\{ \begin{array}{l}x\neq 1\\x\neq 3\end{array} \right.\end{array} \right.$
→ TXĐ là: $D=(-1;+∞)$ \ $\{1;3\}$
e) $\left\{ \begin{array}{l}x-6≥0\\x^2-25 \neq 0\\x+1≥0\end{array} \right.$
$↔ \left\{ \begin{array}{l}x≥6\\\left\{ \begin{array}{l}x\neq 5\\x\neq -5\end{array} \right.\end{array} \right.$
$↔ x≥6$
TXĐ là: $D=[6;+∞)$
