$\begin{array}{l}
a)\,\,y = \,f\left( x \right) = {x^2} - 3x\,\\
\Rightarrow y' = 2x - 3 = 0\\
\Leftrightarrow x = \frac{3}{2}\\
\Rightarrow \left\{ \begin{array}{l}
y\left( 0 \right) = 0\\
y\left( {\frac{3}{2}} \right) = - \frac{9}{4}\\
y\left( 4 \right) = 4
\end{array} \right. \Rightarrow \left\{ \begin{array}{l}
Miny = ..\\
Maxy = ...
\end{array} \right.\\
EM\,\,ket\,\,luan\,\,theo\,\,nua\,\,khoang\,\,cua\,\,em\,\,nhe,\,\,vi\,\,\,voi\,\,x \in \,\,\,\left[ {0;\,\,4} \right)\,\,va\,\,\,\left( {0;\,\,4} \right]\,\,\,co\,\,\,KL\,\,khac\,\,nhau\,\,nhe.\\
b)\,\,\,y = f\left( x \right) = {x^2} - 4x + 3\\
\Rightarrow y' = 2x - 4\\
\Rightarrow y' = 0 \Leftrightarrow 2x - 4 = 0 \Leftrightarrow x = \frac{1}{2}\\
\Rightarrow \left\{ \begin{array}{l}
y\left( { - 2} \right) = 15\\
y\left( {\frac{1}{2}} \right) = \frac{5}{4}\\
y\left( 1 \right) = 0
\end{array} \right.\\
Em\,\,ket\,\,luan\,\,nhe.
\end{array}$
