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Find all angles, \( 0^{\circ} \leq \theta<360^{\circ} \), that solve the following equation. \[ \cos \theta=-\frac{\sqrt{3}}{2} \]

Ask by Ramos Parsons. in the United States
Mar 20,2025

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Answer

The angles that solve the equation are \(150^\circ\) and \(210^\circ\).

Solution

1. We are given the equation \[ \cos \theta = -\frac{\sqrt{3}}{2} \] 2. First, determine the reference angle by ignoring the negative sign: \[ \cos \theta_{\text{ref}} = \frac{\sqrt{3}}{2} \] The angle whose cosine is \(\frac{\sqrt{3}}{2}\) is \(30^\circ\). 3. Since the cosine of \(\theta\) is negative, \(\theta\) must lie in the second or the third quadrant. 4. In the second quadrant, the angle is found by: \[ \theta = 180^\circ - 30^\circ = 150^\circ \] 5. In the third quadrant, the angle is: \[ \theta = 180^\circ + 30^\circ = 210^\circ \] 6. Therefore, the solutions in the interval \(0^\circ \leq \theta < 360^\circ\) are: \[ \theta = 150^\circ \quad \text{and} \quad \theta = 210^\circ. \]

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Beyond the Answer

To solve the equation \(\cos \theta = -\frac{\sqrt{3}}{2}\), we first need to identify the angles where the cosine of an angle is negative. The cosine function is negative in the second and third quadrants. The reference angle associated with \(\cos \theta = \frac{\sqrt{3}}{2}\) is \(30^\circ\) (or \(\frac{\pi}{6}\) radians). Therefore, we need to find the angles corresponding to the reference angle in the second and third quadrants. 1. In the second quadrant: \[ \theta = 180^\circ - 30^\circ = 150^\circ \] 2. In the third quadrant: \[ \theta = 180^\circ + 30^\circ = 210^\circ \] Therefore, the solutions to the equation \(\cos \theta = -\frac{\sqrt{3}}{2}\) in the interval \(0^{\circ} \leq \theta < 360^{\circ}\) are: \[ \theta = 150^\circ, 210^\circ \]

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