解：?$(1)$?設(shè)拋物線相應(yīng)的函數(shù)表達(dá)式為?$y=ax2+bx+c$?
由題意得?$\begin{cases}{4a-2b+c=0 }\\{a+b+c=0} \\{4a+2b+c=8} \end{cases}$? 解得?$\begin{cases}{a=2}\\{b=2}\\{c=-4}\end{cases}$?
則?$y=2x2+2x-4$?
?$(2)$?當(dāng)?$x=-\frac {2a}=-\frac {1}{2}$?時(shí)，?$y=-\frac {9}{2}$?
∴頂點(diǎn)坐標(biāo)為?$(-\frac {1}{2}，$??$-\frac {9}{2})$?
