Câu 11:
\(\Leftrightarrow\left\{{}\begin{matrix}c=6\\-\dfrac{b}{2a}=-2\\4a-2b+c=4\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}b-2b+6=4\\4a=b\\c=6\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{1}{2}\\b=2\\c=6\end{matrix}\right.\\ \Leftrightarrow y=\dfrac{1}{2}x^2+2x+6\\ \text{Chọn }A\)
Câu 12:
\(\Leftrightarrow\left\{{}\begin{matrix}a-b+c=1\\a+b+c=-1\\c=-1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a-b=2\\a+b=0\\c=-1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=1\\b=-1\\c=-1\end{matrix}\right.\\ \Leftrightarrow y=x^2-x-1\\ \text{Chọn }B\)
Câu 13:
\(y=0\Leftrightarrow\left(x+4\right)\left(x+1\right)=0\Leftrightarrow\left[{}\begin{matrix}x=-1\\x=-4\end{matrix}\right.\\ \Leftrightarrow A\left(-1;0\right);B\left(-4;0\right)\text{ là giao của }\left(P\right)\text{ với }Ox\)
