Giải thích các bước giải:
$\begin{array}{l}
a)\left| \alpha \right| \le {360^0}\\
\Leftrightarrow - {360^0} \le \alpha \le {360^0}\\
\Leftrightarrow - {360^0} \le {20^0} + k{360^0} \le {360^0}\\
\Leftrightarrow \dfrac{{ - 19}}{{18}} \le k \le \dfrac{{17}}{{18}}
\end{array}$
Mà $k \in Z \Rightarrow k \in \left\{ { - 1;0} \right\}$
$ \Rightarrow \alpha \in \left\{ { - {{340}^0};{{20}^0}} \right\}$
Vậy $\alpha \in \left\{ { - {{340}^0};{{20}^0}} \right\}$
$\begin{array}{l}
b)\left| \alpha \right| \le {750^0}\\
\Leftrightarrow - {750^0} \le \alpha \le {750^0}\\
\Leftrightarrow - {750^0} \le {20^0} + k{360^0} \le {750^0}\\
\Leftrightarrow \dfrac{{ - 77}}{{36}} \le k \le \dfrac{{73}}{{36}}
\end{array}$
Mà $k \in Z \Rightarrow k \in \left\{ { - 2; - 1;0;1;2} \right\}$
$ \Rightarrow \alpha \in \left\{ { - {{700}^0}; - {{340}^0};{{20}^0};{{380}^0};{{740}^0}} \right\}$
Vậy $\alpha \in \left\{ { - {{700}^0}; - {{340}^0};{{20}^0};{{380}^0};{{740}^0}} \right\}$
