Cho dãy số $\left ( u_{n} \right )$ thoả mãn $\left\{\begin{matrix}u_{1}=4 & \\ u_{n+1}=2u_{n}+\sqrt{3u^{2}_{n}+|1} \end{matrix}\right.$
a. CMR: $u_{n+2}=4u_{n+1}-u_{n}$.
b. CMR: $u_{2015}\vdots 5$