设A（x1,y1）、B(x2,y2),N(x,y),则x=(x1+x2)/2,y=(y1+y2)/2.（1）x1^2/4+y1^2=1x2^2/4+y2^2=1相减得到：（x1^2-x2^2)/4+(y1^2-y2^2)=0,即（x1-x2)(x1+x2)/4+（y1-y2)(y1+y2)=0两边同除（x1-x2),将（1）式代入,化简：...