lhuangblue
幼苗
共回答了15个问题采纳率:86.7% 举报
(1)∵过点P(x,y)的直线分别与x轴和y轴交于A,B两点,点Q与点P关于y轴对称,
∴Q(-x,y),设A(a,0),B(0,b),
∵O为坐标原点,∴
BP =(x,y-b),
PA =(a-x,-y),
OQ =(-x,y),
AB =(-a,b) ,
∵
BP =3
PA 且
OQ •
AB =4 ,
∴
x=3(a-x)
y-b=-3y
ax+by=4 ,
解得点P的轨迹M的方程为
x 2
3 + y 2 =1 .
(2)设过F(2,0)的直线方程为y=kx-2k,
联立
y=kx-2k
x 2
3 + y 2 =1 ,得(3k 2 +1)x 2 -12k 2 x+12k 2 -3=0,
设A(x 1 ,y 1 ),B(x 2 ,y 2 ),则x 1 +x 2 =
12 k 2
3 k 2 +1 ,x 1 x 2 =
12 k 2 -3
3 k 2 +1 ,
FA =(x 1 -2,y 1 ),
FB =(x 2 -2,y 2 ),
∴
FA •
FB =(x 1 -2)(x 2 -2)+y 1 y 2
=(1+k 2 )(x 1 -2)(x 2 -2)
=(1+k 2 )[x 1 x 2 -2(x 1 +x 2 )+4]
=(1+k 2 )(
12 k 2 -3
3 k 2 +1 -
24 k 2
3 k 2 +1 +4)
=
k 2 +1
3 k 2 +1
=
1
3 +
2
9 k 2 +3 ,
∴当k 2 →∞
FA •
FB 的最小值→
1
3 ;当k=0时,
FA •
FB 的最大值为1.
∴
FA •
FB 的取值范围是(
1
3 ,1].
1年前
1