a: \(f\left(x\right)=-x^2+2\left(m+2\right)x-m\left(m+4\right)\)
\(\text{Δ}=\left[2\left(m+2\right)\right]^2-4\cdot\left(-1\right)\cdot\left(-m\right)\left(m+4\right)\)
\(=4\left(m^2+4m+4\right)+4\left(-m^2-4m\right)\)
\(=4m^2+16m+16-4m^2-16m=16>0\)
Để f(x)<=0 với mọi x thuộc R thì \(\left\{{}\begin{matrix}\text{Δ}< =0\\a< 0\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}16< =0\left(sai\right)\\-1< 0\end{matrix}\right.\)
=>\(m\in\varnothing\)