Sequence Term Calculation 2
The sequence $ (a_n) $ is defined by $ a_0 = 2, $ $ a_1 = 1, $ and
\[a_n = a_{n - 1} \sqrt{3} - a_{n - 2}\]for all $ n \ge 2 $. Find $ a_{100} $.
- 1
- 2
- 3
- +
- 4
- 5
- 6
- -
- 7
- 8
- 9
- $\frac{a}{b}$
- .
- 0
- =
- %
- $a^n$
- $a^{\circ}$
- $a_n$
- $\sqrt{}$
- $\sqrt[n]{}$
- $\pi$
- $\ln{}$
- $\log$
- $\theta$
- $\sin{}$
- $\cos{}$
- $\tan{}$
- $($
- $)$
- $[$
- $]$
- $\cap$
- $\cup$
- $,$
- $\infty$
Solution
We have that
\begin{align*}
a_2 &= \sqrt{3} - 2, \\
a_3 &= (\sqrt{3} - 2) \sqrt{3} - 1 = 2 - 2 \sqrt{3}, \\
a_4 &= (2 - 2 \sqrt{3}) \sqrt{3} - (\sqrt{3} - 2) = \sqrt{3} - 4, \\
a_5 &= (\sqrt{3} - 4) \sqrt{3} - (2 - 2 \sqrt{3}) = 1 - 2 \sqrt{3}, \\
a_6 &= (1 - 2 \sqrt{3}) \sqrt{3} - (\sqrt{3} - 4) = -2, \\
a_7 &= (-2) \sqrt{3} - (1 - 2 \sqrt{3}) = -1, \\
a_8 &= (-1) \sqrt{3} - (-2) = 2 - \sqrt{3}, \\
a_9 &= (2 - \sqrt{3}) \sqrt{3} - (-1) = 2 \sqrt{3} - 2, \\
a_{10} &= (2 \sqrt{3} - 2) \sqrt{3} - (2 - \sqrt{3}) = 4 - \sqrt{3}, \\
a_{11} &= (4 - \sqrt{3}) \sqrt{3} - (2 \sqrt{3} - 2) = 2 \sqrt{3} - 1, \\
a_{12} &= (2 \sqrt{3} - 1) \sqrt{3} - (4 - \sqrt{3}) = 2, \\
a_{13} &= 2 \sqrt{3} - (2 \sqrt{3} - 1) = 1.\end{align*}Since $ a_{12} = a_0 = 2 $ and $ a_{13} = a_1 = 1, $ and each term depends only on the previous two terms, the sequence is periodic from here on, with a period of length 12. Hence, $ a_{100} = a_4 = \boxed{\sqrt{3} - 4} $.
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