@Carl Love 

Thank you for the answer.

@Kitonum 

Thank you for the answer.

@Preben Alsholm 

Thank you for the answer.

@Konstantin@ 

a(x) and b(x) are just two functions with x as their variable.

For example arctan(2*x, x^2)=arctan(2*x, x^2) x>0 ? I'm just wondering if this is right and if it applies to all functions. No matter what a(x) and b(x) are.

I'm asking this because I'm trying to understand Maples two-variable arctan function. arctan(y,x).