# Equilateral triangles built on two sides of a triangle

- Author:
- Idan Tal

- Topic:
- Equilateral Triangles, Triangles

The appletillustrates that ifequilateral triangles ABD and ACE are built onthe sides of the triangleABC, then one can see that the size of the angle between BE and CD is fixed and
equals 120

^{º}(invariance property). Another obvious invariantproperty exists concerning the equality of BE and CD.