Why some divisions with a dot product in formulas are not calculate when implementing.

I begin to learn cg shading programming and I try to implement a basic phong model. Within a book that I have, phong model or cook-torrance model, the specular term is divided by some cosine between the light and normal for phond and multiplication of dot(light,normal)xdot(normal,view) for cook-torrance.

However, when I follow the tutorial from CatLikeCoding.com website (or any code on the web) everyone omits those divisions.
why?