**Product rule - Wikipedia**
https://en.wikipedia.org/wiki/Product_rule

• Suppose we want to differentiate f(x) = x sin(x). By using the product rule, one gets the derivative f′(x) = 2x sin(x) + x cos(x) (since the derivative of x is 2x and the derivative of the sine function is the cosine function).• One special case of the product rule is the constant multiple rule, which states: if c is a number and f(x) is a differentiable function, then cf(x) is also differentiable, and its derivative is (cf)′(x) = c f′(x). This follows from the product rule since the derivative of any consta…

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