Prove that: `cos510^0cos 330^0+sin 390^0cos 120^0=-1.`

Explanation

To prove that \( \cos 510^\circ \cos 330^\circ + \sin 390^\circ \cos 120^\circ = -1 \), we will simplify each term step by step.

### Step 1: Simplify \( \cos 510^\circ \)
To simplify \( \cos 510^\circ \), we can reduce it by subtracting \( 360^\circ \):
\[ 510^\circ - 360^\circ = 150^\circ \]
Thus,
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