Work Step by Step
Use the subtraction formula for cosine, $\cos (A-B)=\cos A\cos B+\sin A\sin B$: $\cos(\theta-180^\circ)$ $=\cos\theta\cos 180^\circ+\sin\theta\sin 180^\circ$ $=\cos\theta*(-1)+\sin\theta*0$ $=-\cos\theta$
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