The product rule is a formula used in calculus to find the derivatives of products of two or more functions. It is a fundamental technique for finding the derivative of a function expressed as a product of two functions. The product rule states that the derivative of a product of two functions is equal to the first function times the derivative of the second function plus the second function times the derivative of the first function. In other words, if we have a function f multiplied by another function g, the derivative of f times g is going to be f prime g plus f of g prime. The product rule is more straightforward to memorize than the quotient rule. The formula can be generalized to products of more than two factors.