It is defined as Mf(x)= f(x+ 1)-f(x).

In economics, the derivative function of function (x) is called the marginal function of (x).

It is widely used in engineering, technology, scientific research, national defense, medicine, environmental protection, economy and management and many other fields.

In economics, the cost of producing X product is called cost function, marked as C(X), the income from selling X product is called income function, marked as R(X), and R(X)-C(X) is called profit function, marked as P(X). Accordingly, their derivatives C ′ (x), R ′ (x) and P ′ (x) are called marginal cost function, marginal revenue function and marginal profit function respectively.