Tìm góc lượng giác x sao cho: cos 2x = cos (3x + 10 độ)

cos 2x = cos (3x + 10°)

\( \Leftrightarrow \left[ \begin{array}{l}2x = 3x + 10^\circ + k360^\circ \\2x = - \left( {3x + 10^\circ } \right) + k360^\circ \end{array} \right.\left( {k \in \mathbb{Z}} \right)\)

\( \Leftrightarrow \left[ \begin{array}{l}x = - 10^\circ + k360^\circ \\x = - 2^\circ + k72^\circ \end{array} \right.\left( {k \in \mathbb{Z}} \right)\).