-b^2/a^2.设a（x1,y1)b(x2,y2)斜率为k,om斜率为k1k1=(y1+y2)/(x1+x2)将a,b带入椭圆方程联立可得（y1+y2)/(x1+x2)=-b^2(x2-x1)/a^2/(y2-y1)而（x2-x1)/(y2-y1)刚好为k的倒数所以得出答案啦!好好学习啊,天天向上呵!...