If `A+B+C=pi `, prove that `cos 2A +cos 2B +cos 2C=-1-4cos A cos Bcos C.`
Channel: Doubtnut
Date: 2020-01-03

If `A+B+C=pi `, prove that `cos 2A +cos 2B +cos 2C=-1-4cos A cos Bcos C.`