(i) We know that the angle subtended by an arc of a circle at the centre is double the angle subtended by the arc at any point on the circumference.
So we get
∠AOB = 2 ∠OCA
It is given that ∠AOB = 70o
We can write it as
∠OCA = 70o/2
By division
∠OCA = 35o
Therefore, ∠OCA = 35o.
(ii) From the figure we know that the radius is
OA = OC
We know that the base angles of an isosceles triangle are equal
∠OAC = ∠OCA
It is given that ∠OCA = 35o
So we get
∠OAC = 35o
Therefore, ∠OAC = 35o.