Recall that two circles are congruent if they have the same radii. Prove that equal chords of congruent circles subtend equal angles at their centres.
Prove exactly as Theorem 9.1 by considering chords of congruent circles.
Prove that if chords of congruent circles subtend equal angles at their centres, then the chords are equal.
Use SAS axiom of congruence to show the congruence of the two triangles.