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)

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