Norms or practices. There are many conventional things in mathematics, which are nothing more than convenient calculation, convenient expression, intuitive and unique. For f(x) = A sin (ωx+φ), any ω less than zero can be changed to be greater than zero by coefficient adjustment, and only the sign of a or the size of φ is changed. If written separately, sin(-x) and -sin(x) are generally easier to accept, just like a quadratic equation with one variable always needs to change the first coefficient to a positive value before it is used for factorization. Of course, you can be flexible when solving problems.