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2024 AMC 12A Problem 8

Problem 8 of 25EasierAlgebraGeometryProblem-Solving Techniques

How many angles θ\theta with 0≤θ≤2π0\le\theta\le2\pi satisfy log⁡(sin⁡(3θ))+log⁡(cos⁡(2θ))=0?\log(\sin(3\theta))+\log(\cos(2\theta))=0?

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Solution

The equation means sin⁡(3θ)cos⁡(2θ)=1\sin(3\theta)\cos(2\theta)=1 with both factors positive (for the logs to be defined). Since sin⁡(3θ)≤1\sin(3\theta)\le1 and cos⁡(2θ)≤1,\cos(2\theta)\le1, their product is 11 only if sin⁡(3θ)=1\sin(3\theta)=1 and cos⁡(2θ)=1\cos(2\theta)=1 simultaneously. But cos⁡(2θ)=1\cos(2\theta)=1 forces θ∈{0,π,2π},\theta\in\{0,\pi,2\pi\}, where sin⁡(3θ)=0≠1.\sin(3\theta)=0\ne1. No angle works. Thus, the correct answer is A.
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Tagged: logarithm · trigonometry · bounding to limit cases

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