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Algebra Difficulty 3.0 Junior Find the answer

Given that \[cosθ]=cosθ|\[cos⁡θ]=cos⁡θ, \[tanθ]=tanθ|\[tan⁡θ]=-tan⁡θ, determine the quadrant(s) in which θ2\frac{θ}{2} lies.

Pick one

Solution

[Analysis]
This problem tests the understanding of trigonometric function signs, the representation of quadrant angles, and the application of inequality properties. By analyzing the given conditions, we can determine the quadrant in which θθ lies, and subsequently solve for θ2\frac{θ}{2}.

[Solution]
cosθ=cosθ|cos⁡θ|=cos⁡θ, which means θθ is in the first, fourth quadrant or on the positive half of the xx-axis.
tanθ=tanθ|tan⁡θ|=-tan⁡θ, which means θθ is in the second, fourth quadrant or on the xx-axis.

Therefore, θθ is in the fourth quadrant or on the positive half of the xx-axis.
k360+270<θk360+360k⋅360∘+270∘<θ≤k⋅360∘+360∘, where kZk∈\mathbb{Z}.

Hence, k180+135<θ2k180+180k⋅180∘+135∘<\frac{θ}{2}≤k⋅180∘+180∘, where kZk∈\mathbb{Z}.

Let k=2nk=2n, where nZn∈\mathbb{Z},
Then n360+135<θ2n360+180n⋅360∘+135∘<\frac{θ}{2}≤n⋅360∘+180∘, which is in the second quadrant or on the negative half of the xx-axis.

Let k=2n+1k=2n+1, where nZn∈\mathbb{Z},
Then n360+315<θ2n360+360n⋅360∘+315∘<\frac{θ}{2}≤n⋅360∘+360∘, which is in the fourth quadrant or on the positive half of the xx-axis.

Thus, the answer is D\boxed{D}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.