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Mathematical Expression Editor
Here we compute derivatives of compositions of functions
So far we have seen how to compute the derivative of a function built up from other
functions by addition, subtraction, multiplication and division. There is another very
important way that we combine functions: composition. The chain rule allows
us to deal with this case. Consider While there are several different ways
to differentiate this function, if we let and , then we can express . The
question is, can we compute the derivative of a composition of functions
using the derivatives of the constituents and ? To do so, we need the chain
rule.
Chain Rule If and are differentiable, then
It will take a bit of practice to make the use of the chain rule come naturally, it is
more complicated than the earlier differentiation rules we have seen. Let’s return to
our motivating example.
Compute:
Set and , now Hence
Recall that . We showed this by using the definition of the derivative and the sum of
angles formula. Now that we have the chain rule, we can verify this fact by using the
chain rule.
The derivative of cosine: Take 2
Recall that
.
Now
When working with derivatives of trigonometric functions, we suggest you use
radians for angle measure. For example, while one must be careful with derivatives
as Alternatively, one could think of as meaning , as then . In this case
Let’s see a more complicated chain of compositions.
Compute:
Set and . Hence, and by the chain rule we know Since We have that
The chain rule allows to differentiate compositions of functions that would otherwise
be difficult to get our hands on.
Compute:
set , , and so that . Now
Using the chain rule, the power rule, and the product rule it is possible to avoid using
the quotient rule entirely.
Compute:
Rewriting this as set and so that . Now and by the product and chain rules Since
and , write
We can also compute derivatives with a tables of values.
x
f(x)
f’(x)
g(x)
g’(x)
Using the table of values above, compute when .
Notice that we need to find the derivative of at , but is defined terms of and . Since
is a composition of functions, we need to take the derivative of using the chain rule.
Now that we know what looks like, we want to evaluate at . Using the table of
values, we can see that