b端点的轨迹为（x-2)^2+(y-2√3)^2/4=1
问题等价于过原点且与椭圆相交的直线斜率大小
y=kx 与椭圆方程联立相切时候得最值
(k^2+4)x^2-(4√3k+16)x+24=0
判别式=0得k1=(4√3+2√6)/3
k2=(4√3-2√6)/3
arctank2<=夹角<=arctank1