Điều kiện delta \(\Rightarrow-5\le m\le-1\)
Khi \(-5\le m\le-1\) thì \(0\le\left(m+4\right)^2\le9\)
\(\Rightarrow-\dfrac{9}{2}\le\dfrac{\left(m+4\right)^2-9}{2}\le0\)
\(\Rightarrow0\le\left|\dfrac{\left(m+4\right)^2-9}{2}\right|\le\dfrac{9}{2}\)
Nên \(B_{max}=\dfrac{9}{2}\) khi \(m=-4\)