Two arcs are congruent if the angles made by them at the centre are congruent. This can happen if the arcs lie on the same circle or congruent circles as the radius of these circles will be the same .Congruent circles have the same radius.
In case of same circle and congruent circles the angle subtended by congruent arcs are same.
Two arcs are congruent if they have the same measure and they belong to the same circle or to congruent circles.
