đường thẳng y=m tại ba điểm phân biệt thì tất cả các

* Hướng dẫn giải

Đáp án C

$\begin{array}{l}y\text{'}=3x^2-6x\Rightarrow y\text{'}=0\iff 3x^2-6x=0\\ \end{array}$

$\begin{array}{l}\iff \left[\begin{array}{l}x=0\Rightarrow y_{cd}=1\\ x=2\Rightarrow y_{ct}=-3\end{array}\right.\\ Ycbt\iff y_{cd}<m<y_{cd}.Hay-3<m<1\end{array}$

$$\begin{gathered} \left\{\begin{array}{l}y\text{'}=x^2-2mx+m^2-m+1\\ y\text{'}\text{'}=2x-2m\end{array}\right. \\ \Rightarrow \left\{\begin{array}{l}y\text{'}\left(1\right)=m^2-3m+2=0\\ y\text{'}\text{'}\left(1\right)=2-2m<0\end{array}\right. \\ \Rightarrow \left\{\begin{array}{l}\left[\begin{array}{l}m=1\left(l\right)\\ m=2\left(n\right)\end{array}\right.\\ m>1\end{array}\right.\Rightarrow m=2 \end{gathered}$$