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.
Let ABC be an isosceles triangle with AB = AC.
Triangle ABC has two equal sides: AB and AC. We want to prove that the base angles (angle ABC and angle ACB) are equal.
Take an arbitrary point F on BD (AB extended).
Cut off AG from AE (AC extended) equal to AF.
Join FC and GB.