设A(x1,y1) B(x2,y2)AB中点(x,y) 则x=(x1+x2)/2 y=(y1+y2)/2则直线AB过(0,2):k=(y-2)/xx1^2/2+y1^2=1x2^2/2+y2^2=1(x1-x2)(x1+x2)/2+(y1-y2)(y1+y2)=0所以(x1+x2)/2+(y1+y2)(y1-y2)/(x1-x2)=02x/2+2yk=0x+y(y-2)/x=0x^2+y^2-2y=0这就是弦AB中点P的轨迹方程