Chain rule
If you want the derivative of a function like , you could expand it and use the power rule term-by-term, or rewrite it as a product of three identical factors and use the product rule. Both approaches work, but they get tedious quickly (especially with larger exponents, more complicated polynomials, or fractional powers).
A faster method is designed specifically for composite functions - functions built by plugging one function into another. That method is the chain rule.
Here’s the idea in words:
- Differentiate the outer function, treating the inner function as a single “chunk.”
- Then multiply by the derivative of the inner function.
Another common way to write the chain rule uses Leibniz notation:
Examples
- Differentiate
Solution
is a composite function . The inner function is boxed below:
For clarity,
Outer:
Inner:
- Differentiate the outer function (power rule), keeping the inner function unchanged:
So,
- Differentiate the inner function:
- Multiply the results:
Let’s try one with the natural logarithm. Recall from section 2.5 that .
- Differentiate
Solution
is a composite function with:
- Differentiate the outer function, keeping the inner function unchanged:
- Differentiate the inner function:
- Multiply:
Similarly, recall that for exponential functions,
and
Let’s do 2 examples that use the chain rule.
- Differentiate:
a)
b)
Answers:
a)
b)
Explanations:
a) For , the outer function is and the inner function is .
- Differentiate the outer function:
- Differentiate the inner function:
- Multiply:
b) For , the outer function is and the inner function is .
- Differentiate the outer function:
- Differentiate the inner function:
- Multiply:
Combining all rules
The chain rule often appears together with other derivative rules. The main thing to watch is order: decide which overall structure you have first (product, quotient, or composition), then apply the needed rules inside that structure.
For example, the next problem uses the product rule first, and then uses the chain rule while differentiating one of the factors.
- Differentiate
Solution
is a product of two functions:
Notice that is composite, so the chain rule is needed to find .
| Function | Expression |
|---|---|
Putting these into the product rule:
To simplify, factor out the common factors and :
Contrast that with the next problem. Here, it’s most natural to use the power rule (with the chain rule) first, and then use the product rule when differentiating the inner function.
- Differentiate
Solution
is a composite function with
- Differentiate the outer function:
- Differentiate the inner function:
The inner function is a product, so apply the product rule:
- Multiply:
- Differentiate
Solution
For easier algebra, pull out the constant factor :
Now is a quotient of two functions:
Use the quotient rule. Note that is composite, so the chain rule is needed for .
| Function | Expression |
|---|---|
Putting the pieces into the quotient rule:
You can also rewrite a quotient as a product and then use the product rule, if that feels faster. Using the same function as above,
5b. First turn
into a product of two functions:
and then find the derivative using the product rule.
Solution
As before, keep the constant multiple out in front:
The remaining part is a product of:
Use the product rule. Since is composite, use the chain rule to find .
| Function | Expression |
|---|---|
Now apply the product rule:
Some questions ask you to apply derivative rules using only a table of values:
- and their derivatives have the following values shown in the table:
Evaluate the first derivative of the function
at .
Solution
Answer: -3
Because is a composite function, apply the chain rule:
Now evaluate at :
Challenge problems
- Differentiate .
Solution
Rewrite the square root as a power:
This is a composite function:
For clarity,
Outer:
Inner:
- Differentiate the outer function:
- Differentiate the inner function:
Note: is a constant, so its derivative is . The derivative of is .
- Multiply:
- Differentiate
Solution
Instead of using the quotient rule, rewrite the expression using :
Now apply the chain rule. The outer function is and the inner function is :
This can also be written as
- Differentiate .
Solution
First, rewrite as :
This function has multiple layers of composition:
Outer:
Middle:
Inner:
Apply the chain rule layer by layer: