$解：?(1)?設(shè)?CD?段的函數(shù)表達(dá)式為?y=kx+b?$
$∵?AD=5.5m，??AD：??AC=1：??4?$
$∴?AC=22m?$
$將點(diǎn)?(15，??5.5)、??(37，??0)?代入表達(dá)式得?\begin{cases}{k=-0.25}\\{b=9.25}\end{cases}?$
$∴函數(shù)表達(dá)式為?y=-0.25x+9.25(15≤x≤37)?$
$令?y=4，?則?x=21?$
$?AB=A'B'=21-15=6m?$
$?(2)CD?段的函數(shù)表達(dá)式為?y=-0.25x+9.25(15≤x≤37)?$
$?CD?和?C'D'?關(guān)于?y?軸對稱，$
$則?C'D'?段的函數(shù)表達(dá)式為?y=0.25x+9.25(-37≤x≤-15)?$
$設(shè)?D'FD?段的函數(shù)表達(dá)式為?y=ax^2+8?$
$將點(diǎn)?(15，??5.5)?代入得?a=-\frac {1}{90}?$
$?D'FD?段的函數(shù)表達(dá)式為?y=-\frac {1}{90}x^2+8(-15＜x＜15)?$
$?(3)?在?D'FD?段，令?x=2，??y=\frac {358}{45}≈7.96?$
$?7.96m＞7.4m?$
∴能從橋下安全通行
