Euclid's Elements Book I, Proposition 6:
If in a triangle two angles be equal to one another, the sides which subtend the equal angles will also be equal to one another. Let ABC be a triangle having the angle BAC
equal to the angle ACB;
Geometrical conversion
Proposition I.6 is the geometrical, but not the logical, converse of proposition I.5.
The Elements: Books IXIII

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