3.2.28 ** Let $\left\{f_{n}\right\}$ be the Fibonacci sequence, express $f_{2 n}$ in terms of $f_{n-1}, f_{n}$, and $f_{n+1}$.
Parse: From the previous question, $f_{2 n}=f_{n-1}^{2}+f_{n}^{2}$.

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From the previous question, $f_{2 n}=f_{n-1}^{2}+f_{n}^{2}$.