Evaluate the following trigonometric function at the quadrantal angle, or state that the expression is undefined. \[ \sec 0 \]

The value of \( \sec 0 \) is defined and can be calculated using the relationship between secant and cosine. Since \( \sec \theta = \frac{1}{\cos \theta} \), and \( \cos 0 = 1 \), we find that: \[ \sec 0 = \frac{1}{\cos 0} = \frac{1}{1} = 1. \] So, \( \sec 0 = 1 \). Furthermore, evaluating trigonometric functions at quadrantal angles (like 0, 90, 180, and 270 degrees) often yields simple results, as these angles correspond to key points on the unit circle. In this case, the secant of 0 degrees simply reflects how secant serves as the reciprocal of cosine, neatly tying it back to our understanding of these functions in trigonometric principles!