Solve: `cos3x+cosx-cos2x=0`

Explanation

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

### Step 1: Use the Cosine Addition Formula
We can use the cosine addition formula to combine \( \cos 3x \) and \( \cos x \). The formula for the sum of cosines is:
\[ \cos A + \cos B = 2 \cos\left(\frac{A + B}{2}\right) \cos\left(\frac{A - B}{2}\right) \]
Here, let \( A = 3x \) and \( B = x \).
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