What is the product rule and when do you use it?

The product rule is used when finding the derivative of the product of two functions - two functions multipled together.

If y = f(x)g(x), then

dy/dx = f'(x)g(x) + f(x)g'(x)

Where f'(x) is the derivative of f(x) and g'(x) is the derivative of g(x).