Prove the identity:
(hint: use the laws of cosine)

cos A
= b² + c² - a²
..a.........2abc

Using the above fact, now prove:
(Hint: what would cos B and cos C look like?)
....................b.........c

cos A + cos B + cos C = a² + b² + c²
..a.......b.......c........2abc

2. Originally Posted by adhesive

Prove the identity:
(hint: use the laws of cosine)

cos A = b² + c² - a²
..a.........2abc

Using the above fact, now prove:
(Hint: what would cos B and cos C look like?)
....................b.........c

cos A + cos B + cos C = a² + b² + c²
..a.......b.......c........2abc

The Law of cosines is $a^2 = b^2+c^2-2bc\cos A$

Re-arranging: $-2bc\cos A = a^2-b^2-c^2$

Multiplying both sides by -1: $2bc\cos{A}=b^2+c^2-a^2$

Dividing both sides by 2abc: $\frac{\cos{A}}{a}=\frac{b^2+c^2-a^2}{2abc}$

Similarly, by changing around variables, you can get:

$\frac{\cos B}{b}=\frac{a^2+c^2-b^2}{2abc}$

$\frac{\cos C}{c} = \frac{a^2+b^2-c^2}{2abc}$

So if you simply add them all up:

$\frac{\cos A}{a}+\frac{\cos B}{b} + \frac{\cos C}{c} = \frac{b^2+c^2-a^2}{2abc}+\frac{a^2+c^2-b^2}{2abc}+\frac{a^2+b^2-c^2}{2abc}$

$=\frac{a^2+b^2+c^2}{2abc}$

3. Fantastic, thank you very much!