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If the value of \( \cos \theta=-\frac{\sqrt{3}}{2} \), which of the following could be true?

Ask by Cruz Pollard. in the United States
Dec 03,2024

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Answer

The angles that satisfy \( \cos \theta = -\frac{\sqrt{3}}{2} \) are \( \frac{5\pi}{6} + 2k\pi \) and \( \frac{7\pi}{6} + 2k\pi \), where \( k \) is any integer.

Solution

To determine the possible values of \( \theta \) when \( \cos \theta = -\frac{\sqrt{3}}{2} \), we need to identify the angles where the cosine function takes this value. The cosine function is negative in the second and third quadrants. The reference angle for \( \cos \theta = \frac{\sqrt{3}}{2} \) is \( \theta = \frac{\pi}{6} \) (or \( 30^\circ \)). Therefore, the angles in the second and third quadrants that correspond to \( \cos \theta = -\frac{\sqrt{3}}{2} \) can be found as follows: 1. In the second quadrant: \[ \theta = \pi - \frac{\pi}{6} = \frac{5\pi}{6} \] 2. In the third quadrant: \[ \theta = \pi + \frac{\pi}{6} = \frac{7\pi}{6} \] Thus, the angles that satisfy \( \cos \theta = -\frac{\sqrt{3}}{2} \) are: - \( \theta = \frac{5\pi}{6} + 2k\pi \) for any integer \( k \) (for the second quadrant) - \( \theta = \frac{7\pi}{6} + 2k\pi \) for any integer \( k \) (for the third quadrant) In degrees, these angles correspond to: - \( \theta = 150^\circ + 360^\circ k \) - \( \theta = 210^\circ + 360^\circ k \) Therefore, any angle of the form \( \frac{5\pi}{6} + 2k\pi \) or \( \frac{7\pi}{6} + 2k\pi \) (or their degree equivalents) could be true for \( \cos \theta = -\frac{\sqrt{3}}{2} \).

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The Deep Dive

The value \( \cos \theta = -\frac{\sqrt{3}}{2} \) indicates that \( \theta \) could be in either the second or third quadrant of the unit circle. This means that the possible angles that satisfy this condition are \( \theta = 150^\circ + 360^\circ k \) or \( \theta = 210^\circ + 360^\circ k \), where \( k \) is any integer. This gives potential solutions such as \( \theta = 150^\circ, 210^\circ, 510^\circ, \) and so forth. Additionally, since cosine represents the x-coordinate of a point on the unit circle, this negative value suggests that the angle is such that the projection onto the x-axis is in the negative direction, reaffirming our quadrant deduction. So, the angles should always be considered modulo \( 360^\circ \) for accurate periodic behavior!

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