Let $L_1 = 1, L_2 = 3$ and $L_{n+1} = L_n + L_{n−1}$ for $n ≥ 3$. Prove by induction ...

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$$L_n= \left (1 + \frac{\sqrt{5}}{2}\right)^n + \left (1 − \frac{\sqrt{5}}{2}\right)^n$$

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Lets do it by induction. Clearly, $((1+\sqrt{5})/2)+((1-\sqrt{5})/2) =1=L_1$. Suppose this is true upto $n=k$. Consider,

$L_{n+1}=L_n+L_{n-1}=((1+\sqrt{5})/2)^n+((1-\sqrt{5})/2)^n + ((1+\sqrt{5})/2)^{n-1}+((1-\sqrt{5})/2)^{n-1} = ((1+\sqrt{5})/2)^{n-1}(((1+\sqrt{5})/2)+1) + ((1-\sqrt{5})/2)^{n-1}(((1-\sqrt{5})/2)+1) = ((1+\sqrt{5})/2)^{n-1}((1+\sqrt{5})/2)^2 + ((1-\sqrt{5})/2)^{n-1}((1-\sqrt{5})/2)^2 = ((1+\sqrt{5})/2)^{n+1} + ((1-\sqrt{5})/2)^{n+1} $.