若椭圆ax^2+by^2=1与直线x+y=1交于A,B两点,M为AB的中点直线OM（O为原点）的斜率

设A(x1,y1),B(x2,y2) ,M(x0,y0),把A,B坐标分别代入ax^2+by^2=1把所得两式两边相减,得
a(x1^2-x2^2)+b(y1^2-y2^2)=0,整理得,[(y1+y2)/(x1+x2)]*[(y1-y2)/(x1-x2)]=-a/b,(y0/x0)*kAB=-a/b,
kOM*kAB=-a/b,kAB=-1,直线OM的斜率kOM=a/b