Question 69
Let $\alpha, \beta, \gamma$ be the angles opposite the sides $a, b, c$ of a triangle, respectively. Prove that if the lengths of $a, b, c$ form an arithmetic progression in this order, then the values $\cos \alpha, 1 - \cos \beta, \cos \gamma$ also form an arithmetic progression in that order.
(Otgonbayar Uuye)
(Otgonbayar Uuye)