where \(R\) is the radius of the circumcircle of the triangle \(ABC\).
[You need consider only the case when the angle \(A\) is acute.]
Hence or otherwise deduce that
- \(a \cos A + b \cos B = c \cos (A-B)\),
- \(a \cos A + b \cos B + c \cos C = 4 R \sin A \sin B \sin C\).