Solve: `costheta+cos3theta+cos5theta+cos7theta=0`

Explanation

To solve the equation \( \cos \theta + \cos 3\theta + \cos 5\theta + \cos 7\theta = 0 \), we can follow these steps:

### Step 1: Group the terms
We can group the terms as follows:
\[ (\cos \theta + \cos 7\theta) + (\cos 3\theta + \cos 5\theta) = 0 \]

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