Prove that: `cos 20^0+cos 100^0+cos 140^0=-0`

Explanation

To prove that \( \cos 20^\circ + \cos 100^\circ + \cos 140^\circ = 0 \), we can follow these steps:

### Step 1: Combine \( \cos 20^\circ \) and \( \cos 100^\circ \)
We will use the cosine addition formula:
\[ \cos A + \cos B = 2 \cos\left(\frac{A+B}{2}\right) \cos\left(\frac{A-B}{2}\right) \]
Let \( A = 20^\circ \) and \( B = 100^\circ \).
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