Ответ:

100°

Объяснение:

∠COA = ∠COB

∠AOB = ∠COA + ∠COB

∠COA = ∠AOB:2 = 80:2

∠COA = 40

∠ACO=∠BCO

∠ACB = ∠ACO+∠BCO

∠ACO = 90-∠COA  = 90-40 =50  

∠ACB = 2*50 = 100°