Maths Olympiad Prep

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Algebra Difficulty 4.3 AIME Prove it United States

Problem:
Find all xx between π2-\frac{\pi}{2} and π2\frac{\pi}{2} such that 1sin4xcos2x=1161-\sin^{4} x-\cos^{2} x=\frac{1}{16}.

Solution

Solution:
1sin4xcos2x=116(1616cos2x)sin4x1=016sin4x16sin2x+1=01-\sin^{4} x-\cos^{2} x = \frac{1}{16} \Rightarrow (16-16 \cos^{2} x) - \sin^{4} x - 1 = 0 \Rightarrow 16 \sin^{4} x - 16 \sin^{2} x + 1 = 0.

Use the quadratic formula in sinx\sin x to obtain sin2x=12±34\sin^{2} x = \frac{1}{2} \pm \frac{\sqrt{3}}{4}.

Since cos2x=12sin2x=±32\cos 2x = 1 - 2 \sin^{2} x = \pm \frac{\sqrt{3}}{2}, we get x=±π12,±5π12x = \pm \frac{\pi}{12}, \pm \frac{5\pi}{12}.

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