Practice
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Find an expression for $x_n$ if sequence $\{x_n\}$ satisfies $x_1=2$, $x_2=3$, and
$$
\left\{
\begin{array}{ccll}
x_{2k+1}&=&x_{2k} +x_{2k-1}&\quad (k\ge 1)\\
x_{2k}&=&x_{2k-1} + 2x_{2k-2}&\quad (k\ge 2)
\end{array}
\right.
$$