Suppose we have a function f f defined by y = f (x) y = f (x). When we apply the operator frac {d} {dx} dxd to y y, we have the expression frac {d} {dx}y dxd y, or frac {dy} {dx} dxdy. This expression represents the derivative of y y with respect to x x (note that y y is the function value of f f at x x).