A student will be able to:
- Know the chain rule and its proof.
- Apply the chain rule to the calculation of the derivative of a variety of composite functions.
We want to derive a rule for the derivative of a composite function of the form in terms of the derivatives of and . This rule allows us to differentiate complicated functions in terms of known derivatives of simpler functions.
The Chain Rule
If is a differentiable function at and is differentiable at , then the composition function is differentiable at . The derivative of the composite function is:
Another way of expressing, if and , then
And a final way of expressing the chain rule is the easiest form to remember: If is a function of and is a function of , then
Using the chain rule, let Then
The example above is one of the most common types of composite functions. It is a power function of the type
The rule for differentiating such functions is called the General Power Rule. It is a special case of the Chain Rule.
The General Power Rule
In simpler form, if
What is the slope of the tangent line to the function that passes through point ?
We can write This example illustrates the point that can be any real number including fractions. Using the General Power Rule,
To find the slope of the tangent line, we simply substitute into the derivative:
Find for .
The function can be written as Thus
Let By the chain rule,
This example applies the chain rule twice because there are several functions embedded within each other.
Let be the inner function and be the innermost function.
Using the chain rule,
Notice that we used the General Power Rule and, in the last step, we took the derivative of the argument.
For an introduction to the Chain Rule (5.0), see Khan Academy, Calculus: Derivatives 4: The Chain Rule (9:11).
For more examples of the Chain Rule (5.0), see Math Video Tutorials by James Sousa The Chain Rule: Part 1 of 2 (8:45).
and Math Video Tutorials by James Sousa The Chain Rule: Part 2 of 2 (8:36).