抛物线焦点为F(1,0),设直线AB:y=kx-k,代入y²=4x,得
k²x²-2(k²+2)x+k²=0, 设A（x1,y1）,B(x2,y2）则 x1+x2=2(k²+2)/k², 由焦半径公式,|AB|=|FA|+|FB|=[x1+(p/2)]+[x2+(p/2)]
=(x1+x2)+p=2(k²+2)/k²+2=8, ∴ k²=1, k=±1,倾斜角为α,tanα=k=±1,∵ 0≤α