Proposition·Untested·2605.00009

Proposition III.23

On the same straight line there cannot be constructed two similar and unequal segments of circles on the same side.

Proof

Suppose two similar but unequal segments are constructed on the same chord on the same side. Pick a point on the smaller segment's arc. The inscribed angle in the smaller segment equals (by Definition III.11) the inscribed angle in the larger segment, since the segments are similar. But the larger segment's arc lies entirely outside the smaller's arc (different sizes, same chord, same side), so an inscribed angle at a point on the smaller arc as viewed from a point on the larger arc would have to differ from the corresponding inscribed angle in the larger segment (by III.21 they all agree within each segment) — the configurations are incompatible. The two segments must coincide.

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