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又顶点M(－2，－1) ∴
12AB?1，即AB＝2
∴A(－3，0)，B(－1，0)
将B(－1，0) 代入y?a(x?2)2?1中得a?1
∴抛物线的解析式为y?(x?2)2?1，即y?x2?4x?3 图略
（3）设平行于x轴的直线为y?k
解方程组错误！不能通过编辑域代码创建对象。 得x1??2?k?1，x2??2?k?1 （k??1)
∴线段CD的长为2k?1 ∵以CD为直径的圆与x轴相切 据题意得k?1?k ∴k2?k?1
1?25解得 k?
∴圆心坐标为(?2,
1?25)和(?2,1?25)
91.（08新疆自治区24题）（10分）某工厂要赶制一批抗震救灾用的大型活动板房．如图，板房一面的形状是由矩形和抛物线的一部分组成，矩形长为12m，抛物线拱高为5.6m．
（1）在如图所示的平面直角坐标系中，求抛物线的表达式．
（2）现需在抛物线AOB的区域内安装几扇窗户，窗户的底边在AB上，每扇窗户宽1.5m，高1.6m，相邻窗户之间的间距均为0.8m，左右两边窗户的窗角所在的点到抛物线的水平距离至少为0.8m．请计算最多可安装几扇这样的窗户？
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（08新疆自治区24题解析）24．（10分）解：（1）设抛物线的表达式为y?ax2 1分 点B(6，?5.6)在抛物线的图象上． ∴?5.6?36a
a??745 ······························································ 3分
745x ············································································ 4分
2∴抛物线的表达式为y??（2）设窗户上边所在直线交抛物线于C、D两点，D点坐标为（k，t）
已知窗户高1.6m，∴t??5.6?(?1.6)??4 ·························································· 5分
?4??745k
2k1≈5.07，k2≈?5.07（舍去） ··········································································· 6分
∴CD?5.07?2≈10.14（m）·············································································· 7分 又设最多可安装n扇窗户
∴1.5n?0.8(n?1)≤10.14 ····················································································· 9分
n≤4.06．
答：最多可安装4扇窗户．····················································································10分 （本题不要求学生画出4个表示窗户的小矩形）
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