The electrical code in my area requires using GFCI outlet for outlets within a certain distance of water source. I don't remember precisely the distance (and it likely vary a bit by jurisdiciton), but i know that in my bathroom, there's nowhere beyond it.
In order to use that sort of outlet in a bathroom like mine, it would have to have GFCI protection as well as USB, be a second outlet wired to the GFCI protected terminal of a GFCI outlet, or be wired to a circuit breaker with GFCI protection built in, in the electrical panel.
I'm just being a pendant here, but the derivative of x²+C is 2x. You put the constant at the wrong place.
Also, i'm glad you found a textbook well suited for you. I have to wonder what you mean by 'why', do you mean a proof?