Problem

Source: Canada RepĂȘchage 2021/7 CMOQR

Tags: Canada, repechage, algebra, trigonometry



If $A, B$ and $C$ are real angles such that $$\cos (B-C)+\cos (C-A)+\cos (A-B)=-3/2,$$find $$\cos (A)+\cos (B)+\cos (C)$$