They're not. Unfortunately, we cannot make every write mathematics correctly (I've tried!) so we have to guess what they really mean!
but is different.
For example, if x= 1 then sin(x)= sin(1)= 0.84147, approximately, so that while [tex]sin(x^2)= sin(1)= 0.84147[tex].
It is a function notation. As such, it must be written in function notation: .
It is quite often the case that the notation is used in place of .
In several computer algebra systems, one must use for they will not recognize .
But again, reread my post. I told to use () with the sine function.