Find all $a,$ $0^\circ < a < 360^\circ,$ such that $\cos a,$ $\cos 2a,$ and $\cos 3a$ form an arithmetic sequence, in that order.  Enter the solutions, separated by commas, in degrees.
Question:
Find all $a,$ $0^\circ < a < 360^\circ,$ such that $\cos a,$ $\cos 2a,$ and $\cos 3a$ form an arithmetic sequence, in that order.  Enter the solutions, separated by commas, in degrees.
Answer:
Let's think step by step.

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45^\circ, 135^\circ, 225^\circ, 315^\circ
Find all $a,$ $0^\circ < a < 360^\circ,$ such that $\cos a,$ $\cos 2a,$ and $\cos 3a$ form an arithmetic sequence, in that order.  Enter the solutions, separated by commas, in degrees.
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