\[\sin A+\sin B+\sin C=4\cos \frac{A}{2}\cos\frac{B}{2}\cos\frac{C}{2}\]
\[\cos A+\cos B+\cos C=1+4\sin\frac{A}{2}\sin\frac{B}{2}\sin\frac{C}{2}\]
\[\cos^2 A+\cos^2 B+\cos^2 C+2\cos A\cos B \cos C =1\]
原文：https://www.cnblogs.com/Ano-Ano/p/12209847.html