It seems to me that brackets are randomly thrown around when talking Sin, Cos and Tan.
For example, now I have a problem in my book that states an equation: how am I to understand this?
As or ??
From experience, arguments of trigonometric functions often have the form , so I would say it's . However, omitting parentheses does not give credit to the authors of the book. See this thread by an engineer with a Ph.D. turned high school teacher.