九年级数学中考压轴题典型题型解析
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2009年全国中考数学压轴题精选精析
37.(09年黑龙江牡丹江)28.(本小题满分8分)
如图,ABCD 在平面直角坐标系中,6AD =,若OA 、OB 的长是关于x 的一元二
次方程2
7120x x -+=的两个根,且OA OB >. (1)求sin ABC ∠的值.
(2)若E 为x 轴上的点,且16
3
AOE S =
△,求经过D 、E 两点的直线的解析式,并判断AOE △与DAO △是否相似?
(3)若点M 在平面直角坐标系内,则在直线AB 上是否存在点F ,使以A 、C 、F 、M
为顶点的四边形为菱形?若存在,请直接写出F 点的坐标;若不存在,请说明理由.
(09年黑龙江牡丹江28题解析)解:(1)解2
7120x x -+=得1243x x ==,
OA OB >
43OA OB ∴==, ·
············································································· 1分 在Rt AOB △
中,由勾股定理有5AB =
4
sin 5
OA ABC AB ∴∠=
= ·
······································································· 1分 (2)∵点E 在x 轴上,16
3
AOE S =△
11623
AO OE ∴⨯= 8
3OE ∴=
880033E E ⎛⎫⎛⎫∴- ⎪ ⎪⎝⎭⎝⎭
,或, ········································································ 1分
由已知可知D (6,4)
设DE y kx b =+,当8
03E ⎛⎫ ⎪⎝⎭
,
时有
28题图
46803k b k b =+⎧⎪⎨
=+⎪⎩解得65165k b ⎧=⎪⎪⎨⎪=-⎪⎩ ∴616
55
DE y x =
- ·
··············································································· 1分 同理803
E ⎛⎫
- ⎪⎝⎭
,
时,616
1313
DE y x =+ ·
······················································ 1分 在AOE △中,8
9043
AOE OA OE ∠===°,, 在AOD △中,9046OAD OA OD ∠===°,, OE OA
OA OD
=
AOE DAO ∴△∽△ ·
··········································································· 1分 (3)满足条件的点有四个
123475224244(38)(30)1472525F F F F ⎛⎫⎛⎫
----- ⎪ ⎪⎝⎭⎝⎭
,;,;,;, ····························· 4分
说明:本卷中所有题目,若由其它方法得出正确结论,可参照本评分标准酌情给分.
38.(09年黑龙江齐齐哈尔)28.(本小题满分10分) 直线3
64
y x =-
+与坐标轴分别交于A B 、两点,
动点P Q 、同时从O 点出发,同时到达A 点,运动停止.点Q 沿线段OA 运动,速度为每秒1个单位长度,点P 沿路线O →B →A 运动.
(1)直接写出A B 、两点的坐标;
(2)设点Q 的运动时间为t 秒,OPQ △的面积为S ,求出S 与t 之间的函数关系式; (3)当48
5
S =
时,求出点P 的坐标,并直接写出以点O P Q 、、为顶点的平行四边形的第四个顶点M 的坐标.
(09年黑龙江齐齐哈尔28题解析)(1)A (8,0)B (0,6) ································· 1分 (2)86OA OB ==, 10AB ∴=
点Q 由O 到A 的时间是
8
81
=(秒) ∴点P 的速度是
610
28
+=(单位/秒) ·
1分 当P 在线段OB 上运动(或03t ≤≤)时,2OQ t OP t ==,
2S t = ·········································································································· 1分
当P 在线段BA 上运动(或38t <≤)时,6102162OQ t AP t t ==+-=-,, 如图,作PD OA ⊥于点D ,由
PD AP BO AB =
,得4865
t
PD -=, ······························ 1分 21324255
S OQ PD t t ∴=⨯=-+ ·
······································································ 1分 (自变量取值范围写对给1分,否则不给分.)
(3)82455P ⎛⎫ ⎪⎝⎭
, ···························································································· 1分
12382412241224555555I M M 2⎛⎫⎛⎫⎛⎫-- ⎪ ⎪ ⎪⎝⎭⎝⎭⎝⎭
,,,,, ···················································· 3分
注:本卷中各题,若有其它正确的解法,可酌情给分. 39.(09年黑龙江绥化)28.(本小题满分lO 分)
(09年黑龙江绥化28题解析)