Đáp án:
Giải thích các bước giải:
`A(x)=ax^2+bx+c`
`A(2)=a.2^2+b.2+c=0`
`=>4a+2b+c=0`
`A(-1)=a.(-1)^2+b.(-1)+c=2`
`=>a-b+c=2`
`A(1)=a.1^2+b.1+c=1`
`=>a+b+c=1`
Ta có
$\left\{ \begin{array}{l}4a+2b+c=0\\ a-b+c=2\\ a+b+c=1\end{array} \right.$
`(a-b+c)+(a+b+c)=2+1`
`=>2(a+c)=3`
`=>a+c=3/2`
Ta có
$\left\{ \begin{array}{l}4a+2b+c=0\\ a-b+c=2\\ a+b+c=1\end{array} \right.$
`=>`$\left\{ \begin{array}{l}4a+2b+c=0\\ 2a-2b+2c=4\\ a+b+c=1\end{array} \right.$
`=>(4a+2b+c)+(2a-2b+2c)=0+4`
`=>6a+3c=4`
`=>2a+c=4/3`
Ta có
$\left\{ \begin{array}{l}a+c=\dfrac{3}{2}\\ 2a+c=\dfrac{4}{3}\end{array} \right.$
`=>`$\left\{ \begin{array}{l}a=\dfrac{-1}{6}\\ c=\dfrac{5}{3}\end{array} \right.$
`=>b=-1/2`
Vậy `a=-1/6,b=-1/2,c=5/3`
