过点A(-1,0)斜率为k的直线方程
y=k(x+1)代入 y^2=4x 得
k^2x^2+2k^2x+k^2=4x
k^2x^2+(2k^2-4)x+k^2=0
x1+x2=-(2k^2-4)/k^2=3
k=正负2根5/5