Skip to main content

2024 AMC 12A Problem 8

Problem 8 of 25EasierAlgebraGeometry

How many angles θ\theta with 0θ2π0\le\theta\le2\pi satisfy log(sin(3θ))+log(cos(2θ))=0?\log(\sin(3\theta))+\log(\cos(2\theta))=0?

Answer choices

Show solution

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)=01.\sin(3\theta)=0\ne1. No angle works. Thus, the correct answer is A.

More practice

Concepts: logarithm · trigonometry · bounding to limit cases

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.