Nếu x , α là góc lượng giác có đơn vị là rađian thì A. cos x = cos α ⇔ x = α + k 2 π , k ∈ Z . B. cos x = cos α ⇔ x = − α + k 2 π , k ∈ Z .

\(\cos x = \cos \alpha \Leftrightarrow x = \alpha + k2\pi ,k \in \mathbb{Z}.\)

\(\cos x = \cos \alpha \Leftrightarrow x = - \alpha + k2\pi ,k \in \mathbb{Z}.\)

\(\cos x = \cos \alpha \Leftrightarrow \left[ {\begin{array}{*{20}{l}}{{\rm{x}} = \alpha + {\rm{k}}2\pi }\\{{\rm{x}} = \pi - \alpha + k2\pi }\end{array},k \in \mathbb{Z}} \right..\)

\(\cos x = \cos \alpha \Leftrightarrow \left[ {\begin{array}{*{20}{l}}{{\rm{x}} = \alpha + {\rm{k}}2\pi }\\{{\rm{x}} = - \alpha + {\rm{k}}2\pi }\end{array},{\rm{k}} \in \mathbb{Z}} \right..\)