Answer:
∠BCO = 70°
Step-by-step explanation:
AO = CO = BO because all radii of the same circle are equal.
∠ACO = 20° because it is an isosceles triangle.
∠BCA = 90° because a triangle with it's hypotenuse being the diameter of a circle is a right triangle.
∠BCA - ∠ACO = ∠BCO
90° - 20° = 70°
∠BCO = 70°
