a: Theo đề, ta có hệ:
\(\left\{{}\begin{matrix}a\cdot1^2+b\cdot1+2=5\\a\cdot\left(-2\right)^2+b\cdot\left(-2\right)+2=8\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a+b=3\\4a-2b=6\end{matrix}\right.\)
=>a=2; b=1
b: Theo đề, ta có hệ:
\(\left\{{}\begin{matrix}0^2+b\cdot0+c=-4\\2^2+b\cdot2+c=-6\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}c=-4\\2b+4-4=-6\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}b=-3\\c=-4\end{matrix}\right.\)
c; Theo đề, ta có hệ:
\(\left\{{}\begin{matrix}a\cdot2^2+b\cdot2+1=2\\\dfrac{-b}{2a}=\dfrac{1}{2}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}4a+2b=1\\b=-a\end{matrix}\right.\)
=>4a+2b=1 và a+b=0
=>a=1/2; b=-1/2
