The law of product (also called Leibniz's law) is the law of calculating the derivative of the product of two functions in mathematics. From this, many derivative formulas of other products are derived (some formulas need to be mastered by rote).

For example, two continuous functions f, g and their derivatives f', g' are known, and the derivative of their product fg is: (fg)'= fg)'= f'g+fg''.

Let u=u(x) and v=v(x), then

(uv)' = u'v+uv '，

This is the derivative formula of multiplication.

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Not all functions have derivatives, and a function does not necessarily have derivatives at all points. If the derivative of a function exists at a certain point, it is said to be derivative at this point, otherwise it is called non-derivative. However, the differentiable function must be continuous; Discontinuous functions must be non-differentiable.

For differentiable function f(x), x? F'(x) is also a function called the derivative function of f(x). The process of finding the derivative of a known function at a certain point or its derivative function is called derivative. Derivative is essentially a process of finding the limit, and the four algorithms of derivative also come from the four algorithms of limit. Conversely, the known derivative function can also be used to find the original function, that is, indefinite integral.

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