代入
(x-2)^2=ax^2
x^2-(4+a)x+4=0
x1+x2=a+4,x1x2=4
y=x-2
所以y1y2=(x1-2)(x2-2)=x1x2-2(x1+x2)+4=-2a
OA斜率y1/x1,OB斜率y2/x2
垂直(y1/x1)(y2/x2)=-1
y1y2=-x1x2
-2a=-4
a=2