$$angle$$BAC = 85° $$angle$$BCA = 75° => $$angle$$ABC = 180°-(85°+75°) = 20° Angle subtended by an arc at the centre is twice the angle subtended by it at any point on the circle. => $$angle$$AOC = 2$$angle$$ABC => $$angle$$AOC = 40° In $$ riangle$$OAC, OA = OC = radii => $$angle$$OAC = $$angle$$OCA => 2$$angle$$OAC = 180°-40° = 140° => $$angle$$OAC = $$frac{140^{circ}}{2}$$ = 70°