Solve: `cos3x+cosx-cos2x=0`

Explanation

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

### Step 1: Use the Cosine Addition Formula
We can use the cosine addition formula to combine the terms. The formula states:
\[ \cos A + \cos B = 2 \cos\left(\frac{A+B}{2}\right) \cos\left(\frac{A-B}{2}\right) \]
In our case, let \( A = 3x \) and \( B = x \):
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