解：如圖建立平面直角坐標(biāo)系
$設(shè)拋物線形橋拱相應(yīng)的二次函數(shù)表達式為?y=ax^2(-\frac {15}{2}≤x≤\frac {15}{2})?$
$則由題意，點?(\frac {15}{2}，??-7)?在拋物線上$
$∴?-7=a×(\frac {15}{2})^2，??a=-\frac {28}{225}?$
$∴拋物線相應(yīng)的函數(shù)表達式為?y=-\frac {28}{225}x^2(-\frac {15}{2}≤x≤\frac {15}{2})?$
