Để tam thức ko đổi dấu trên R

\(\Leftrightarrow\left\{{}\begin{matrix}m-1>0\\\Delta'=m^2-\left(m-1\right)\left(3m-2\right)< 0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}m>1\\-2m^2+5m-2< 0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}m>1\\\left[{}\begin{matrix}m< \frac{1}{2}\\m>2\end{matrix}\right.\end{matrix}\right.\) \(\Rightarrow m>2\)

\(f\left(x\right)=\left(m+2\right)x^2+2\left(m+2\right)x+m+3>0\) ∀x

\(\Leftrightarrow\left\{{}\begin{matrix}a>0\\\Delta'< 0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}m+2>0\\\left(m+2\right)^2-\left(m+2\right)\left(m+3\right)< 0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}m>-2\\m^2+4m+4-m^2-5m-6< 0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}m>-2\\-m-2< 0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}m>-2\\m>-2\end{matrix}\right.\)

Vậy m>-2

Để tam thức ko đổi dấu trên R

\(\Leftrightarrow\Delta'< 0\)

\(\Leftrightarrow\left(m+1\right)^2-\left(m^2+2\right)^2< 0\)

\(\Leftrightarrow\left(m^2+m+3\right)\left(-m^2+m-1\right)< 0\)

\(\Leftrightarrow\left(m^2+m+3\right)\left(m^2-m+1\right)>0\) (luôn đúng)

Vậy với mọi m thì \(f\left(x\right)>0\)

\(\Leftrightarrow\left\{{}\begin{matrix}a< 0\\\Delta< 0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}m-2< 0\\\left(m-3\right)^2-\left(m-2\right)\left(m-1\right)< 0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}m< 0\\m^2-6m+9-m^2+3m-2< 0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}m< 0\\-3m+7< 0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}m< 0\\m>\frac{7}{3}\end{matrix}\right.\) (vô lý)

=> Ko tồn tại m t/m đề bài

\(f\left(x\right)=x^2-\left(m+2\right)x+8m+1>0\) với mọi x

\(\Leftrightarrow\left\{{}\begin{matrix}a>0\\\Delta< 0\end{matrix}\right.\)

\(\Leftrightarrow\left(m+2\right)^2-4\left(8m+1\right)< 0\)

\(\Leftrightarrow m^2+4m+4-32m-4< 0\)

\(\Leftrightarrow m^2-28m< 0\)

\(\Leftrightarrow0< m< 28\)

\(f\left(x\right)=x^2+4x+\left(m-2\right)^2>0\) ∀x

\(\Leftrightarrow\left\{{}\begin{matrix}a>0\\\Delta'< 0\end{matrix}\right.\)

\(\Leftrightarrow4-\left(m-2\right)^2< 0\)

\(\Leftrightarrow4-m^2+4m-4< 0\)

\(\Leftrightarrow-m^2+4m< 0\)

\(\Leftrightarrow\left[{}\begin{matrix}x< 0\\x>4\end{matrix}\right.\)

Để tam thức ko đổi dấu trên R

\(\Leftrightarrow\left\{{}\begin{matrix}m+4\ne0\\\Delta< 0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}m\ne-4\\25+16\left(m+2\right)< 0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}m\ne-4\\16m+57< 0\end{matrix}\right.\)

\(\Rightarrow\left\{{}\begin{matrix}m\ne-4\\m< -\frac{57}{16}\end{matrix}\right.\)

\(f\left(x\right)=\left(2-m\right)x^2-2x+1>0\) ∀x

\(\Leftrightarrow\left\{{}\begin{matrix}a>0\\\Delta< 0\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}2-m>0\\1-2+m< 0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}m< 2\\m< 1\end{matrix}\right.\Leftrightarrow m< 1\)

\(f\left(x\right)=5x^2-x+m>0\text{ ∀x}\)

\(\Leftrightarrow\left\{{}\begin{matrix}a>0\\\Delta< 0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}5>0\left(lđ\right)\\1-4.5.m< 0\end{matrix}\right.\)

\(\Leftrightarrow1-20m< 0\Leftrightarrow m>\frac{1}{20}\)

Các câu dạng như này làm tương tự nhé bạn

Cách làm:

\(f\left(x\right)=ax^2+bx+c\)

\(f\left(x\right)>0\text{ ∀x}\Leftrightarrow\left\{{}\begin{matrix}a>0\\\Delta< 0\end{matrix}\right.\)