b^2=1c^2=8a^2=9椭圆方程x^2/9+y^2=1将直线y=x+210x^2+18x-27=0两根为x1.， x2 y1-y2=x1-x2中点横坐标=(x1+x2)/2=-9/10中点纵坐标=(x1+2+x2+2)/2=11/10
