设过M的弦AB的中点为PA(x1,y1),B(x2,y2),P(x,y)则 x=(x1+x2)/2,y=(y1+y2)/2 (1)且 (y2-y1)/(x2-x1)=(y-1)/x (2)将AB坐标代入椭圆方程得x1^2+y1^2/4=1x2^2+y2^2/4=1两式相减得(x2+x1)(x2-x1)+(y2+y1)(y2-y1)/4=0把（1...