>>149
Okay, but dx is not d times x, it is a single value, isn't it? Otherwise you could simplify the fraction and get 1/x and the differential would be just f(x)/x or y/x or whatever. So what is the d? Is it an infinitesimal value? Do you just multiply d with f(x), like the notation implies?
For higher order derivatives, there's also the notation \frac{d^n}{dx^n}??? How does any of this make sense?