# Prove that triangle is equilateral

• Jun 12th 2013, 09:22 AM
feferon11
Prove that triangle is equilateral
Over the sides of an arbitrary triangle ABC from the outside are constructed isosceles triangles with an angle of $120^o$ at the top.
Prove that the three vertices of this triangles form an equilateral triangle (triangle DFE on picture).

Angles: $AFC=AEB=BDC= 120^o$

Thank you.
• Jun 12th 2013, 05:28 PM
chiro
Re: Prove that triangle is equilateral
Hey feferon11.

Hint: Can you prove that all interior angles are 60 degrees?
• Jun 13th 2013, 01:41 PM
johng
Re: Prove that triangle is equilateral
Hi,
Your problem is a rather famous theorem, namely Napoleon's Theorem. There are close to a zillion proofs of this theorem. You can start at wikapedia's article to start examining the various proofs. I've attached a sketch and a brief indication why your problem is Napoleon's.

Attachment 28614
• Jun 13th 2013, 10:51 PM
ibdutt
Re: Prove that triangle is equilateral
Chiro has given a very good hint.
angle FCD = 120 given. triangle FCD is an isosceles triangle thus angle CFD = angle CFD = 30 degree. similarly from triangle EAF we will get
angle AEF = angle AFE = 30 degree
We have angle AFC = 120
I am sure now you can establish that angle EFD = 60 degree and similarly other angles.
Quote:

Originally Posted by feferon11
Over the sides of an arbitrary triangle ABC from the outside are constructed isosceles triangles with an angle of $120^o$ at the top.
Prove that the three vertices of this triangles form an equilateral triangle (triangle DFE on picture).

Angles: $AFC=AEB=BDC= 120^o$

Thank you.