Solve that equation : `costheta+cos3theta-2cos2theta=0`

Explanation

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

### Step 1: Use the Cosine Addition Formula
We know that:
\[ \cos A + \cos B = 2 \cos\left(\frac{A + B}{2}\right) \cos\left(\frac{A - B}{2}\right) \]
In our case, let \( A = \theta \) and \( B = 3\theta \). Thus, we can rewrite the left-hand side:
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