Có lấy nhé
$\begin{array}{l} \sin x = - 1\\ \Leftrightarrow \sin x = \sin \left( { - {{90}^o}} \right)\\ \Leftrightarrow \left[ \begin{array}{l} x = - {90^o} + k{360^o}\\ x = {180^o} - \left( { - {{90}^o}} \right) + k{360^o} \end{array} \right.\\ \Leftrightarrow \left[ \begin{array}{l} x = - {90^o} + k{360^o}\\ x = {270^o} + k{360^o} \end{array} \right.\\ \Leftrightarrow x = - {90^o} + k{360^o} \end{array}$
