# If $n$ is an integer which leaves remainder one when divided by three, then $(1+\sqrt {3}i)^n+(1-\sqrt {3}i)^n=$

$\begin {array} {1 1} (1)\;-2^{n+1} & \quad (2)\;2^{n+1} \\ (3)\;-(-2)^n & \quad (4)\;-2^n \end {array}$

$(3)\;-(-2)^n$