a) \({{ - 1} \over 2}\) = cos \({{2\pi } \over 3}\) nên cos x = \({{ - 1} \over 2}\) ⇔ cos x = cos \({{2\pi } \over 3}\)
⇔ x = ±\({{2\pi } \over 3}\) + k2π, k ∈ Z
b)cos x = \({2 \over 3}\) ⇒ x = ± arccos \({2 \over 3}\) + k2π, k ∈ Z
c) \({{\sqrt 3 } \over 2}\) = cos30o nên cos(x + 30o )= \({{\sqrt 3 } \over 2}\)
⇔ cos(x + 30o ) = cos 30o
⇔ x + 30o = ±30o + k360o, k ∈ Z
⇔ x = k360o, k ∈ Z và x = -60o + k360o, k ∈ Z
