设点A(0,m),易知B(-c,0)所以点D(-c/2,m/2),由于点D在椭圆上故有:c^2/4a^2+m^2/4b^2=1
m^2=b^2*(4-e^2),所以AB=a*(2-e^2),cos∠ABC=c/AB=e/(2-e^2)