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Algebra Difficulty 4.1 AIME Find the answer United States

How many angles θ\theta with 0θ2π0 \le \theta \le 2\pi satisfy log(sin(3θ))+log(cos(2θ))=0\log(\sin(3\theta)) + \log(\cos(2\theta)) = 0?

Pick one

Solution

Suppose θ\theta satisfies the equation. Then log(sin(3θ)cos(2θ))=0\log(\sin(3\theta) \cdot \cos(2\theta)) = 0. This implies sin(3θ)cos(2θ)=1\sin(3\theta) \cdot \cos(2\theta) = 1, so applying the Product-to-Sum Formula
sinacosb=12(sin(a+b)+sin(ab)) \sin a \cdot \cos b = \frac{1}{2} (\sin(a + b) + \sin(a - b))
gives 12(sin(5θ)+sinθ)=1\frac{1}{2}(\sin(5\theta) + \sin\theta) = 1. Then sin(5θ)+sinθ=2\sin(5\theta) + \sin\theta = 2, so sin(5θ)=sinθ=1\sin(5\theta) = \sin\theta = 1, and the only possible solution is θ=π2\theta = \frac{\pi}{2}.
However, this solution is not valid because sin(3θ)\sin(3\theta) is equal to 1-1, and log(sin(3θ))\log(\sin(3\theta)) is not defined.

As in the first solution, sin(3θ)cos(2θ)=1\sin(3\theta) \cdot \cos(2\theta) = 1. Then either sin(3θ)=1\sin(3\theta) = 1 and cos(2θ)=1\cos(2\theta) = 1, or sin(3θ)=1\sin(3\theta) = -1 and cos(2θ)=1\cos(2\theta) = -1. However, only the first case is possible because otherwise the logarithms in the equation are not defined. In the first case, 3θ{π2,5π2,9π2}3\theta \in \{\frac{\pi}{2}, \frac{5\pi}{2}, \frac{9\pi}{2}\} and 2θ{0,2π,4π}2\theta \in \{0, 2\pi, 4\pi\}, yielding θ{π6,5π6,3π2}\theta \in \{\frac{\pi}{6}, \frac{5\pi}{6}, \frac{3\pi}{2}\} and θ{0,π,2π}\theta \in \{0, \pi, 2\pi\}. The two sets have no common values for θ\theta, so the first case yields no solutions.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.