Inverse trigonometric functions, such as the inverse cosine, serve to reverse the action of their corresponding trigonometric functions. While a standard cosine function takes an angle and returns a cosine value, the inverse cosine function starts with a cosine value and returns the angle. This relationship is crucial when evaluating expressions like the inverse cosine of \( \frac{1}{2} \).
To find the angle corresponding to a cosine value, we can utilize the unit circle. For instance, when evaluating \( \cos^{-1} \left( \frac{1}{2} \right) \), we seek the angle \( \theta \) such that \( \cos(\theta) = \frac{1}{2} \). On the unit circle, we identify that \( \theta = \frac{\pi}{3} \) in the first quadrant has a cosine value of \( \frac{1}{2} \). Additionally, \( \theta = \frac{5\pi}{3} \) in the fourth quadrant also yields a cosine value of \( \frac{1}{2} \). However, only one of these angles is valid for the inverse cosine function.
To determine the correct angle, we must consider the defined range of the inverse cosine function, which is limited to angles between \( 0 \) and \( \pi \). This restriction ensures that the inverse cosine function remains a valid function, as the cosine function is not one-to-one over its entire range. By reflecting only the segment of the cosine graph from \( 0 \) to \( \pi \) over the line \( y = x \), we obtain the graph of the inverse cosine function.
Given this interval, we can conclude that \( \frac{5\pi}{3} \) is not a valid solution since it falls outside the specified range. Therefore, the only valid angle for \( \cos^{-1} \left( \frac{1}{2} \right) \) is \( \frac{\pi}{3} \). In summary, when evaluating inverse cosine expressions, it is essential to find the corresponding angle on the unit circle while adhering to the defined interval of \( 0 \) to \( \pi \).