In any ΔABC, prove that 2(bc cos A + ca cos B + ab cos C) = (a 2 + b 2 + c 2 )

Question

Philoid · Accepted Answer

Need to prove: 2(bc cos A + ca cos B + ab cos C) = (a² + b² + c²)

Left hand side

2(bc cos A + ca cos B + ab cos C)

b² + c² – a² + c² + a² – b² + a² + b² – c²

a² + b² + c²

Right hand side. [Proved]