Question

Giúp em câu 4 với ạ

Answer 1

cosA/a+cosB/b+cosC/c

\(=\dfrac{b^2+c^2-a^2}{2\cdot abc}+\dfrac{c^2+a^2-b^2}{2bac}+\dfrac{a^2+b^2-c^2}{2abc}\)

\(=\dfrac{b^2+c^2-a^2+c^2+a^2-b^2+a^2+b^2-c^2}{2bac}\)

\(=\dfrac{b^2+a^2+c^2}{2abc}\)