Given f(x(t), y(t)), I know that ∂f/∂x and ∂f/∂y is true (with ∂) because, by definition, use ∂ where f is function of 2 (x, y) or more variable (x, y, z)... ok! But, which theory explain the use of d or ∂ when derive f with respect to parameter t? Is df/dt or ∂f/∂t? And if the function is f(x(t, s), y(t, s)), so, the correct is df/dt or ∂f/∂t? Why? Why? Why? Thanks!