Because triangle BDE is folded by triangle BDA.

So triangle BDE is equal to triangle BDA.

So BE=BA angle bed = angle a = 60.

Because BE=AC+CE=AC+CF=AF.

So BA=AF

So the triangle ABF is an equilateral triangle.

So BF=AB=BE angle F = 60.

Because BE=BF BC=BC CE=CF

So triangle BCF is equal to triangle BCE.

So angle BEC= angle f = 60.

So the angle dec = 120.

So BEC angle = 1/2 angle CED.