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92. \( \cos \theta=-\frac{1}{4}, \quad \tan \theta>0 \)

Ask by Wang Robbins. in the United States
Jan 24,2025

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\(\theta\) is in Quadrant III, with \(\sin \theta = -\frac{\sqrt{15}}{4}\) and \(\tan \theta = \sqrt{15}\).

Solución

Given the conditions: \[ \cos \theta = -\frac{1}{4}, \quad \tan \theta > 0 \] Let's analyze these to determine the quadrant in which angle \(\theta\) lies and find relevant trigonometric values. ### 1. Determining the Quadrant - **\(\cos \theta = -\frac{1}{4}\)**: - The cosine of an angle is negative in **Quadrants II** and **III**. - **\(\tan \theta > 0\)**: - The tangent of an angle is positive when both sine and cosine have the **same sign**. - Since \(\cos \theta\) is negative, for \(\tan \theta\) to be positive, \(\sin \theta\) must also be negative. **Conclusion**: Both sine and cosine are negative, which occurs in **Quadrant III**. ### 2. Finding \(\sin \theta\) Using the Pythagorean identity: \[ \sin^2 \theta + \cos^2 \theta = 1 \] \[ \sin^2 \theta = 1 - \cos^2 \theta = 1 - \left(-\frac{1}{4}\right)^2 = 1 - \frac{1}{16} = \frac{15}{16} \] \[ \sin \theta = -\sqrt{\frac{15}{16}} = -\frac{\sqrt{15}}{4} \] (The negative sign is chosen because \(\sin \theta\) is negative in Quadrant III.) ### 3. Finding \(\tan \theta\) \[ \tan \theta = \frac{\sin \theta}{\cos \theta} = \frac{-\frac{\sqrt{15}}{4}}{-\frac{1}{4}} = \sqrt{15} \] ### Summary - **Quadrant**: III - **\(\sin \theta\)**: \(-\frac{\sqrt{15}}{4}\) - **\(\tan \theta\)**: \(\sqrt{15}\) If you have a specific aspect of the problem you'd like to explore further, feel free to ask!

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Bonus Knowledge

To find the angle \( \theta \) given that \( \cos \theta = -\frac{1}{4} \) and \( \tan \theta > 0 \), we first note that since \( \cos \theta \) is negative, \( \theta \) must be in the second or third quadrant. However, \( \tan \theta > 0 \) indicates that \( \theta \) is in the third quadrant, where both sine and cosine are negative. Using the cosine value, we can find the sine using the Pythagorean identity \( \sin^2 \theta + \cos^2 \theta = 1 \): \[ \sin^2 \theta + \left(-\frac{1}{4}\right)^2 = 1 \implies \sin^2 \theta + \frac{1}{16} = 1 \] \[ \sin^2 \theta = 1 - \frac{1}{16} = \frac{15}{16} \implies \sin \theta = -\sqrt{\frac{15}{16}} = -\frac{\sqrt{15}}{4} \] The sign is negative because \( \theta \) is in the third quadrant. Now, we have \( \cos \theta = -\frac{1}{4} \) and \( \sin \theta = -\frac{\sqrt{15}}{4} \). If we need to find \( \theta \), we can use the tangent relation: \[ \tan \theta = \frac{\sin \theta}{\cos \theta} = \frac{-\frac{\sqrt{15}}{4}}{-\frac{1}{4}} = \sqrt{15}. \] Thus, you can find the reference angle \( \theta = \pi + \tan^{-1}(\sqrt{15}) \) for the exact value. The final answer for \( \theta \) in radians is: \[ \theta = \pi + \tan^{-1}(\sqrt{15}). \]

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