Question

求证三角形中,COS(B-C)-COS(B+C)

Answer 1

证明：∵A+B+C=180°

∴B+C=180°-A

∴-COS(B+C)=-COS(180°-A)=COSA

∵COS(B-C)≤1

∴COS(B-C)-COS(B+C)≤1+COSA