4x^2+y^2=4
y=x+2
消y得
4x^2+x^2+4x+4=4
5x^2+4x=0
x1=0 y1=2
x2=-5/4 y2=3/4
弦AB=√[(x1-x2)^2+(y1-y2)^2]=√[4+4]=2√2把x^2+y^2/4=1改为x^2-y^2/4=1呢？