设斜率为2的弦与椭圆交于A（x1,y1),B(x2,y2),x1^2/2+y1^2=1,(1),x2^2/2+y2^2=1,(2),(1)-(2)式,（x1^2-x2^2)/2+(y1^2-y^2)=0,(x1+x2)(x1-x2)/2+(y1-y2)(y1+y2)=0,[(y1-y2)/(x1-x2)]*[(y1+y2)/2]/[(x1+x2)/2]=-1/2,其...