Lời giải:
ĐKXĐ: \(\left\{\begin{matrix} 2-x\geq 0\\ 2x+m\geq 0\end{matrix}\right.\Leftrightarrow \left\{\begin{matrix} x\leq 2\\ x\geq \frac{-m}{2}\end{matrix}\right.\) \(\Leftrightarrow x\in [\frac{-m}{2}; 2]\)

Để TXĐ có độ dài $1$ thì:

\(2-\frac{-m}{2}=1\Leftrightarrow m=-2\)

ĐKXĐ: \(\left\{{}\begin{matrix}2x-m+1\ge0\\m+3-x\ge0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}x\ge\frac{m-1}{2}\\x\le m+3\end{matrix}\right.\)

Hàm có TXĐ bằng rỗng khi và chỉ khi:

\(\frac{m-1}{2}>m+3\Leftrightarrow m< -7\)

\(\left\{{}\begin{matrix}m\le x\\x\le3\end{matrix}\right.\Rightarrow m\le3\Rightarrow\left[m;3\right]\) 

Vay \(m\le3\) thi ham so co tap xd la 1 doan tren truc so

P/s: Ve cai truc so ra la hieu

a.

\(\Leftrightarrow x^2+2\left(m-1\right)x+m^2+3m+5\ne0\) ; \(\forall x\)

\(\Leftrightarrow\Delta'=\left(m-1\right)^2-\left(m^2+3m+5\right)< 0\)

\(\Leftrightarrow-5m-4< 0\)

\(\Leftrightarrow m>-\dfrac{4}{5}\)

b. 

\(\Leftrightarrow x^2+2\left(m-1\right)x+m^2+m-6\ge0\) ;\(\forall x\)

\(\Leftrightarrow\Delta'=\left(m-1\right)^2-\left(m^2+m-6\right)\le0\)

\(\Leftrightarrow-3m+7\le0\)

\(\Rightarrow m\ge\dfrac{7}{3}\)

c.

\(x^2-2\left(m+3\right)x+m+9>0\) ;\(\forall x\)

\(\Leftrightarrow\Delta'=\left(m+3\right)^2-\left(m+9\right)< 0\)

\(\Leftrightarrow m^2+5m< 0\Rightarrow-5< m< 0\)