Verified answer

arcCos²x - 8arcCosx + 15 = 0

arcCosx = m ,   0 ≤ m ≤ π

m² - 8m + 15 = 0

m₁ = 3      m₂ = 5

1) arcCosx = 3         3 ∈ [0 , π]

x = Cos3

2) arcCosx = 5        5 ∉ [0 ; π]

x = Cos5

Ответ : Cos3