What is equivalent to cos 150 degrees?

The cosine of 150 degrees is equivalent to the cosine of 180 degrees minus 30 degrees, which can be expressed as (-\cos(30^\circ)). Since (\cos(30^\circ) = \frac{\sqrt{3}}{2}), it follows that (\cos(150^\circ) = -\frac{\sqrt{3}}{2}). Therefore, (\cos 150^\circ = -\frac{\sqrt{3}}{2}).