In equal circles equal straight lines cut off equal circumferences, the greater equal to the greater and the less to the less.
Proof
Let AB and CD be equal chords in equal circles with centres E,
F. In △ABE and △CDF: AB=CD, AE=BE=CF=DF (radii, equal circles). By SSS (I.8) the triangles are
congruent and ∠AEB=∠CFD. By III.26 the arcs are
equal. The corresponding major arcs (complements in the equal
circles) are also equal.