$$\left. \begin{array} { l } { 30 ^ { \circ } } \\ { | A B | = 20 m , A } \\ { \text { e v e l \right.$$
$A\in \mathrm{R}\text{, }B=0\text{, }m=0\text{, }a=30\text{, }b=A$
$A=0\text{, }B\neq 0\text{, }m=0\text{, }a=30\text{, }b=0$
$A=\frac{20m}{|B|}\text{, }B=\frac{20m}{b}\text{, }m>0\text{, }a=30\text{, }b>0$
$A=\frac{20m}{|B|}\text{, }B=-\frac{20m}{b}\text{, }m>0\text{, }a=30\text{, }b>0$
$A=-\frac{20m}{|B|}\text{, }B=-\frac{20m}{b}\text{, }m>0\text{, }a=30\text{, }b<0$
$A=-\frac{20m}{|B|}\text{, }B=\frac{20m}{b}\text{, }m>0\text{, }a=30\text{, }b<0$
