Calculate the measure of ∠AOC.
120∘
70∘
140∘
90∘
Join AC. In ΔAOB,
OA = OB (radii of same circle)
∠OAB=∠OBA=40∘
In ΔOBC,
OC = OB (radii of same circle)
∠OCB=∠OBC=30∘
Now,
∠ABC=∠ABD+∠OBC
=40+30=70∘
∠AOC=2∠ABC=2×70=140∘