Work Step by Step
Use the subtraction formula for cosine, $\cos (A+B)=\cos A\cos B-\sin A\sin B$: $\cos(180^\circ+\theta)$ $=\cos 180^\circ\cos \theta-\sin 180^\circ\sin \theta$ $=-1*\cos\theta-0*\sin\theta$ $=-\cos\theta$
You can help us out by revising, improving and updating this answer.Update this answer
After you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.