Definition 11

Similar segments of circles are those which admit equal angles, or in which the angles equal one another.


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Since the segments BGC and EHF have in them equal angles BGC and EHF, this definition declares them to be similar segments.

This is hardly a proper definition considering that proposition III.21 has yet to be proved in which it is shown that all the angles in one segment are equal.

Besides III.21, similar segments are mentioned in III.24 and III.26.