Which expression is єquivalent to \( \cos 120^{\circ} ? \) \( \cos 60^{\circ} \) \( \cos 240^{\circ} \) \( \cos 300^{\circ} \) \( \cos 420^{\circ} \)

Cosine is a periodic function, so we can find equivalent angles by adding or subtracting multiples of \(360^{\circ}\). The cosine of \(120^{\circ}\) can be expressed as \(-\cos 60^{\circ}\), since \(120^{\circ}\) is in the second quadrant. Therefore, it is equivalent to \(\cos 240^{\circ}\), which is also \(-0.5\). For a quick way to remember cosine values for specific angles, you can think of the unit circle! Cosine values represent the x-coordinates of points on the circle. \(120^{\circ}\) is in the second quadrant, where x-values are negative (thus giving \(-\frac{1}{2}\)), while points like \(60^{\circ}\) (first quadrant) and \(240^{\circ}\) (third quadrant) can help reinforce your understanding of these relationships.