- [Maximum mark: 7]
Quadrilateral OABC is shown on the following set of axes. A figure shows a quadrilateral OABC on a Cartesian plane with O at the origin, A on the positive x‑axis, and C in the first quadrant. The quadrilateral is symmetric about the line segment [OB].

(a) (i) Write down the coordinates of the midpoint of [AC].
(ii) Hence or otherwise, find the equation of the line passing through the points O and B.
(b) Given that [OA] is perpendicular to [AB], find the area of the quadrilateral OABC.