Concept: Product rule (editing)

When taking the derivative of a product with $n$ factors, the product rule gives that this derivative is equal a sum of $n$ terms, the $i$th of which is the original function but with the $i$th factor replaced by the derivative of the $i$th factor: $\frac{d}{dt}(xy)=\frac{dx}{dt} y+x \frac{dy}{dt}$