9. 乙题:

解:(1)因为反比例函数的图象经过点

,················································································································ 2分

.····················································································································· 3分

所以反比例函数的解析式为,············································································· 4分

(2)当为一、三象限角平分线与反比例函数图像的交点时,

线段最短.············································································································ 5分

代入,解得,即.····················· 6分

,··········································································································· 7分

,··········································································································· 8分

为反比例函数图像上的任意两点,

由图象特点知,线段无最大值,即.·················································· 9分

7. 解:(1)设药物燃烧阶段函数解析式为,由题意得:

························································································································ 2分

此阶段函数解析式为······································································· 3分

(2)设药物燃烧结束后的函数解析式为,由题意得:

·························································································································· 5分

此阶段函数解析式为······································································ 6分

(3)当时,得···················································································· 7分

························································································································· 8分

·························································································································· 9分

从消毒开始经过50分钟后学生才可回教室.···························································· 10分

4. 解:(1)由题意可知,

解,得 m=3.     ………………………………3分

A(3,4),B(6,2);

k=4×3=12.   ……………………………4分

(2)存在两种情况,如图: 

①当M点在x轴的正半轴上,N点在y轴的正半轴

上时,设M1点坐标为(x1,0),N1点坐标为(0,y1).

∵ 四边形AN1M1B为平行四边形,

∴ 线段N1M1可看作由线段AB向左平移3个单位,

再向下平移2个单位得到的(也可看作向下平移2个单位,再向左平移3个单位得到的).

由(1)知A点坐标为(3,4),B点坐标为(6,2),

N1点坐标为(0,4-2),即N1(0,2);    ………………………………5分

M1点坐标为(6-3,0),即M1(3,0).    ………………………………6分

设直线M1N1的函数表达式为,把x=3,y=0代入,解得

∴ 直线M1N1的函数表达式为. ……………………………………8分

②当M点在x轴的负半轴上,N点在y轴的负半轴上时,设M2点坐标为(x2,0),N2点坐标为(0,y2). 

ABN1M1ABM2N2ABN1M1ABM2N2

N1M1M2N2N1M1M2N2.  

∴ 线段M2N2与线段N1M1关于原点O成中心对称.   

M2点坐标为(-3,0),N2点坐标为(0,-2).   ………………………9分

设直线M2N2的函数表达式为,把x=-3,y=0代入,解得

∴ 直线M2N2的函数表达式为.   

所以,直线MN的函数表达式为.  ………………11分

(3)选做题:(9,2),(4,5).  ………………………………………………2分

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