Product Rule

Definition

The product rule is a fundamental principle in calculus used to find the derivative of the product of two functions. It states that if you have two differentiable functions, say $$u(x)$$ and $$v(x)$$, then the derivative of their product is given by $$ (uv)' = u'v + uv' $$. This rule highlights the relationship between the functions and their derivatives, making it essential when dealing with multi-variable functions where partial derivatives are involved.

5 Must Know Facts For Your Next Test

1. The product rule is particularly useful when differentiating expressions that involve multiple functions multiplied together.
2. When applying the product rule, you must differentiate each function separately and then combine them according to the formula.
3. This rule can be extended to more than two functions; for three functions $$u, v, w$$, the derivative is given by $$ (uvw)' = u'vw + uv'w + uvw' $$.
4. In the context of partial derivatives, if $$z = f(x,y)g(x,y)$$, you would apply the product rule with respect to each variable while treating the other as a constant.
5. It's important to ensure that each function involved in the product rule is differentiable in the domain of interest to avoid undefined derivatives.

Review Questions

• How does the product rule relate to finding partial derivatives of a function that is defined as the product of two other functions?
  • When dealing with a function defined as the product of two other functions, such as $$z = f(x,y)g(x,y)$$, you apply the product rule to find its partial derivatives with respect to both variables. For instance, when finding $$\frac{\partial z}{\partial x}$$, you would differentiate $$f$$ while keeping $$g$$ constant and vice versa. This illustrates how the product rule integrates into the broader context of partial derivatives, allowing for more complex relationships between variables.
• In what scenarios would you need to use the product rule instead of just applying basic differentiation rules?
  • You would need to use the product rule when you are faced with differentiating an expression that consists of two or more functions multiplied together. For example, if you have a function like $$h(x) = (x^2 + 1)(sin(x))$$, using basic differentiation rules alone would not yield the correct result since it's not a simple sum or single function. The product rule allows you to accurately calculate the derivative by considering both parts of the multiplication.
• Evaluate how the application of the product rule might change if you're working with vector-valued functions rather than scalar-valued functions.
  • When applying the product rule to vector-valued functions, such as $$ extbf{F}(t) = extbf{u}(t) extbf{v}(t)$$ where both $$ extbf{u}$$ and $$ extbf{v}$$ are vector functions of time $$t$$, you'll still follow the same principle as for scalar functions. However, you must consider that each component of the vector may have different dependencies on time. Thus, applying the product rule results in a vector whose components are derived from applying the product rule to each corresponding component function. This highlights how differentiation in higher dimensions requires careful consideration of all interactions between component functions.

Related terms

Partial Derivative: A partial derivative represents the derivative of a function with respect to one variable while keeping other variables constant.

Chain Rule: The chain rule is a formula for computing the derivative of the composition of two or more functions.

Total Derivative: The total derivative considers all possible ways a function can change with respect to changes in its variables, encompassing both direct and indirect effects.