Proof: By Euclid

(related to Corollary: A Criterion for Isosceles Triangles)

"\(\Rightarrow\)": Let a triangle be isosceles. Then, by virtue of Prop. 1.5, the angles at its base are equal.

"\(\Leftarrow\)": Let the angles at the base of a triangle be equal. Then, by virtue of Prop 1.6 it is isosceles.

Thank you to the contributors under CC BY-SA 4.0!



Adapted from CC BY-SA 3.0 Sources:

  1. Callahan, Daniel: "Euclid’s 'Elements' Redux" 2014

Adapted from (Public Domain)

  1. Casey, John: "The First Six Books of the Elements of Euclid"