A.2x+y-8=0 B.2x-y+8=0

C.2x+y-12=0 D.2x-y-12=0

解析:点M的坐标为(2,4),点N的坐标为(3,2),由两点式方程得 (y"-" 2)/(4"-" 2)=(x"-" 3)/(2"-" 3),即2x+y-8=0.

答案:A

5.已知点M(1,-2),N(m,2),若线段MN的垂直平分线的方程是 x/2+y=1,则实数m的值是(　　)

A.-2 B.-7 C.3 D.1

解析:由中点坐标公式,得线段MN的中点坐标是((1+m)/2 "," 0).又点((1+m)/2 "," 0)在线段MN的垂直平分线上,所以 (1+m)/4+0=1,所以m=3,故选C.

答案:C

6.经过点(0,3),且在两坐标轴上截距之和等于5的直线方程是　　　　　.

解析:设直线方程为 x/a+y/b=1,则{■(b=3"," @a+b=5"," )┤

解得{■(a=2"," @b=3"," )┤则直线方程为 x/2+y/3=1,

即3x+2y-6=0.

答案:3x+2y-6=0

7.已知直线l经过点P(-1,2),与x轴、y轴分别相交于A,B两点.若P为线段AB的中点,则直线l的方程为　　　　　.

解析:设A(x,0),B(0,y).

由P(-1,2)为AB的中点,

∴{■((x+0)/2="-" 1"," @(0+y)/2=2"," )┤　∴{■(x="-" 2"," @y=4"." )┤

由截距式得l的方程为