3cos6x - 8cos3x +5=0
3(2Сos²3x -1) - 8cos3x +5 = 0
6Cos²3x - 3 - 8cos3x +5 = 0
6Cos²3x -8Cos3x +2 = 0
Cos3x = t
6t² - 8t +2 = 0
t = (4 +-√(16 -12))/6 = (4 +-2)/6
а) t₁ = 1 б) t₂ = 1/3
Cos3x = 1 Cos3x = 1/3
3x = 2πk , k ∈Z 3x = +-arcCos1/3 + 2πn , n ∈Z
x = 2πk/3 , k ∈Z x = +-1/3*arcCos1/3 + 2πn/3 , n ∈Z