抛物线y=2x2上两点A(x1,y1),B(x2,y2)关于直线y=x+m对称,若2x1x2=-1,则2m的值是(  ) A.3 B.4 C.5 D.6

问题描述:

抛物线y=2x2上两点A(x1,y1),B(x2,y2)关于直线y=x+m对称,若2x1x2=-1,则2m的值是(  )
A. 3
B. 4
C. 5
D. 6

y1y2
x1x2
=−1,
y1+y2
2
x1+x2
2
+m,2x1x2=−1,以及y1=2x12,y2=2x22 可得
 x2x1y1y2=2(
x 21
x 22
),⇒x1+x2=−
1
2

2m=(y1+y2)−(x1+x2)=2(
x 21
+
x 22
)+
1
2
=2(x1+x2)2−4x1x2+
1
2
=2•
1
4
+2+
1
2
=3

故选 A.