x+2y=0
x=-2y代入圆方程得
(-2y)^2+y^2-2*(-2y)+4y-4=0
4y^2+y^2+4y+4y-4=0
5y^2+8y-4=0
y1+y2=-8/5y1y2=-4/5
(y1-y2)^2=(y1+y2)^2-4y1y2
=(-8/5)^2-4*(-4/5)
=64/25+16/5
=144/25

(x1-x2)^2=(-2y1+2y2)^2=4(y1-y2)^2=4*144/25=576/25

所以弦长=√[(x1-x2)^2+(y1-y2)^2]
=√(576/25+144/25)
=12√5 / 5