Solve: `cos3theta+2costheta=0`

Explanation

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

### Step 1: Use the identity for \( \cos 3\theta \)
We know that:
\[ \cos 3\theta = 4\cos^3 \theta - 3\cos \theta \]
Substituting this into the equation gives:
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