Question 15
Given real number $\alpha$, please find the minimum real number $\lambda = \lambda(\alpha)$, such that for any complex numbers $z_1, z_2$ and real number $x \in [0, 1]$, if $|z_1| \le \alpha |z_1 - z_2|$ then $|z_1 - xz_2| \le \lambda |z_1 - z_2|$. (posed by Li Shenghong)
