Solve `cos theta+cos 3 theta-2 cos 2 theta=0`.

Explanation

To solve the equation \( \cos \theta + \cos 3\theta - 2 \cos 2\theta = 0 \), we can follow these steps:

### Step 1: Rewrite the equation
We start with the equation:
\[ \cos \theta + \cos 3\theta - 2 \cos 2\theta = 0 \]

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