Writes that AB = BC, as they are tangents from an external point to a circle.
Notes that OA = OC as they are radius.
Writes that ∠BAO = ∠BCO = 90° as AB and BC are tangents.
Notes that OA || BC as ∠AOC + ∠OCB = 180° (adjacent interior angles)
Notes that OC || AB as ∠AOC + ∠OAB = 180° (adjacent interior angles)
Concludes that OABC is a parallelogram.
Writes that, as opposite sides in a parallelogram are equal, OA = BC and OC = AB.
Also, as opposite angles in a parallelogram are equal, ∠AOC = ∠ABC = 90
(Award full marks if students first proves that OABC is a rectangle using angle sum property and then shows that the adjacent sides are equal.)
Concludes that OABC is a square as all of its angles are 90°, and OA = AB = BC = OC.
