Trigonometric Integer Solution
Find the smallest positive integer solution to $ \tan{19x^{\circ}}=\dfrac{\cos{96^{\circ}}+\sin{96^{\circ}}}{\cos{96^{\circ}}-\sin{96^{\circ}}} $.
- 1
- 2
- 3
- +
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- 5
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- -
- 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
By the tangent addition formula,
\begin{align*}
\frac{\cos 96^\circ + \sin 96^\circ}{\cos 96^\circ - \sin 96^\circ} &= \frac{1 + \tan 96^\circ}{1 - \tan 96^\circ} \\
&= \frac{\tan 45^\circ + \tan 96^\circ}{1 - \tan 45^\circ \tan 96^\circ} \\
&= \tan (45^\circ + 96^\circ) \\
&= \tan 141^\circ.\end{align*}Thus, we seek the smallest positive integer solution to
\[\tan 19x^\circ = \tan 141^\circ.\]This means $ 19x - 141 = 180n $ for some integer $ n, $ or $ 19x - 180n = 141 $. We can use the Extended Euclidean Algorithm to find the smallest positive integer solution.
Running the Euclidean Algorithm on 180 and 19, we get
\begin{align*}
180 &= 9 \cdot 19 + 9, \\
19 &= 2 \cdot 9 + 1, \\
9 &= 9 \cdot 1.\end{align*}Then
\begin{align*}
1 &= 19 - 2 \cdot 9 \\
&= 19 - 2 \cdot (180 - 9 \cdot 19) \\
&= 19 \cdot 19 - 2 \cdot 180.\end{align*}Multiplying both sides by 141, we get
\[2679 \cdot 19 - 282 \cdot 180 = 141.\]Note that if $ (x,n) $ is a solution to $ 19x - 180n = 141, $ then so is $ (x - 180,n + 19) $. Thus, we reduce 2679 modulo 180, to get $ x = \boxed{159} $.
Alternatively, we want to solve
\[19x \equiv 141 \pmod{180}.\]Multiplying both sides by 19, we get
\[361x \equiv 2679 \pmod{180},\]which reduces to $ x \equiv \boxed{159} \pmod{180} $.