Let the side length of the cube be 1, then DC= 1 and ED= 1/2 are both in CDE of RT triangle. ∠D=90 degrees, e is the midpoint of AD,

According to pythagorean theorem CE 2 = DE 2+CD 2

Introduce data. CE=√5/2

∠ ced = de/EC = ∠ 5/5 cosine.