If the chord is perpendicular to OD, the hypotenuse of the right triangle AOD is 3, and 3, AD is 3/2. 3. According to Pythagorean theorem, OD = 3/2 and AOD = 60 degrees can be obtained.

Central angle AOB = 2 AOD = 120 degrees.

Circumferential angle = half of central angle = 60 degrees.