二切线相互垂直,则其倾斜角分别为π/4和3π/4, 斜率分别为1和-1；方程分别为y = x-a²/c和y = -x +a²/c; 与圆心(原点)的距离均为半径a
因为二者关于x轴对称,可以只取一个计算,不妨取前者,y = x - a²/c, x -y - a²/c = 0
与圆心(原点)的距离d = r = |-a²/c|/√2 = a
e = c/a = 1/√2 = √2/2