Compute $\cos (-150^\circ)$.
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Compute $\cos (-150^\circ)$.
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
-\frac{\sqrt{3}}{2}
Compute $\cos (-150^\circ)$.
train