Let a, b, c be the sides of any triangle ABC. Then by applying the sine rule, we get

⇒ a = k sin A, b = k sin B, c = k sin C

So by considering the LHS of the given equation, we get

Substituting the corresponding values from sine rule, we get

Rearranging we get

Regrouping this we get

But

Hence the above equation becomes,

(by applying sine rule)

Hence proved