Question

If $\angle BOA={210}^{\circ }$ and $\angle BOC={120}^{\circ }$, find $\angle COA.$

• A. ${60}^{\circ }$
• B. ${30}^{\circ }$
• C. ${90}^{\circ }$
• D. ${150}^{\circ }$

Answer 1

The correct option is C ${90}^{\circ }$

$\angle BOC$ and $\angle COA$ are adjacent angles with common vertex, O and common arm, CO.

$\because \angle BOC$ and $\angle COA$ are adjacent angles, $\angle BOA=\angle BOC+\angle COA$

$\Rightarrow \angle COA=\angle BOA-\angle BOC$
$=210-120={90}^{\circ }$