Euclid Book 1 - Proposition 5
Isosceles triangle base angles are equal

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To be proved: In isosceles triangles the angles at the base equal one another, and, if the equal straight lines are produced further, then the angles under the base equal one another. This proof is known as Pons Asinorum, or Bridge of Asses.

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There is a triangle ABC with equal sides AB and AC extended to AD and AE.

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