Often Mathematics books are just a little vague as happens when something is so clear to the author that the author has no idea an explanation is needed. In other cases things are stated once, and if you do not pay attention to the details you can later find yourself asking how did we get to this point later. Trigonometry has several simple ideas that can be daunting if we just miss those few words, or made simple if we just understand a little deeper.
Another problem occurs in all of mathematics. Definitions are often wordy, and one might wonder why all the extra words that seem to be there just to clutter the idea being presented. Unfortunately, those words are usually needed, albeit for cases that might not occur at the level the material is intended.
The solution is a few clarifying words can be added to help make things understandable.