(a) What is meant by the perpendicular distance from the centre of a circle to a chord?
(b) A circle has two chords AB and CD such that AB=CD. Explain, without performing any calculation, why the perpendicular distances from the centre to AB and to CD must be equal.
(c) If a third chord EF is closer to the centre than AB, explain which of the two chords, AB or EF, is longer and give a reason for your answer.