Prove that: `cos 20^0cos 100^0+cos 100^0cos 140^0-\ cos 140^0cos 200^0=-3/4`

Explanation

To prove that:

\[ \cos 20^\circ \cos 100^\circ + \cos 100^\circ \cos 140^\circ - \cos 140^\circ \cos 200^\circ = -\frac{3}{4} \]

we will follow these steps:

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