1996 AIME Problems/Problem 10

Problem

Find the smallest positive integer solution to $\tan{19x^{\circ}}=\dfrac{\cos{96^{\circ}}+\sin{96^{\circ}}}{\cos{96^{\circ}}-\sin{96^{\circ}}}$ .

Solution

$\dfrac{\cos{96^{\circ}}+\sin{96^{\circ}}}{\cos{96^{\circ}}-\sin{96^{\circ}}} =$ $\dfrac{\sin{186^{\circ}}+\sin{96^{\circ}}}{\sin{186^{\circ}}-\sin{96^{\circ}}} =$ $\dfrac{\sin{(141^{\circ}+45^{\circ})}+\sin{(141^{\circ}-45^{\circ})}}{\sin{(141^{\circ}+45^{\circ})}-\sin{(141^{\circ}-45^{\circ})}} =$ $\dfrac{2\sin{141^{\circ}}\cos{45^{\circ}}}{2\cos{141^{\circ}}\sin{45^{\circ}}} = \tan{141^{\circ}}$ .

The period of the tangent function is $180^\circ$ , and the tangent function is one-to-one over each period of its domain.

Thus, $19x \equiv 141 \pmod{180}$ .

Since $19^2 \equiv 361 \equiv 1 \pmod{180}$ , multiplying both sides by $19$ yields $x \equiv 141 \cdot 19 \equiv (140+1)(18+1) \equiv 0+140+18+1 \equiv 159 \pmod{180}$ .

Therefore, the smallest positive solution is $x = \boxed{159}$ .

Solution 2

$\dfrac{\cos{96^\circ}}+\sin{96^{\circ}}}{\cos96^{\circ}}-\sin{96^{\circ}}} = \dfrac{1+\tan{96^{\circ}}}{1-\tan{96^{\circ}}} = \drac{\tan{45^{\circ}}+\tan{96^{\circ}}}{1-\tan{45^{\circ}}\tan{96^{\circ}}} = \tan{141^{\circ}}$ (Error compiling LaTeX. Unknown error_msg)

So $19x = 141 +180n$ , for some integer $n$ . Then $0 \equiv 8 + 9n \pmod{19}$ , from which $n \equiv 16 \pmod{19}$ after some manipulation. The smallest suitable value of $n$ is therefore 16 from which $x = \boxed{159}$

1996 AIME (Problems • Answer Key • Resources)
Preceded by Problem 9	Followed by Problem 11
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15
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1996 AIME Problems/Problem 10

Contents

Problem

Solution

Solution 2

See also