Question

in fig.2 o is the centre of the circle. find angle ADB

Answer 1

140° since the angle formed at the centre is double that at the rest of circle furthermore ACBD is a cyclic quadrilateral

Answer 2

∠O=80°
reflex∠O= 360-80 = 280
Since angle subtended by a chord at the center is double the angle subtended by it on any point on the circle, 
∠D = (1/2)reflex∠O
      = 280/2
      = 140°
∴∠D = 140°