Answer
$m\widehat {AB}=90^{\circ}$
Work Step by Step
The measure of a central angle equals the measure of its central arc. Thus,
$m\widehat {BC}=m\angle AOB=90^{\circ}$.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.