Construction: Extend BC to cut the circle at E and join AE.
∆AEB is right angled at E. (Angle in a semicircle is a right angle). Hence, both AE and OD are perpendicular to EB. By AA property: ∆AEB -∆ODB Hence,
OB
AB
=
OD
AE
or
OD
AE
=
1
2
⇒ AE = 2OD = 3cm In right ∆AEC: ∠ACE = 180° ∠ACB - 180° -120° = 60° and AC=