To understand the chain rule, we first need to define what a composite function is. A composite function is a function that is made up of two or more functions applied in sequence, where the output of one function becomes the input of the next. For example, if we have two functions f(x) and g(x), we can create a composite function h(x) by applying f(x) to g(x), such that h(x) = f(g(x)).
Now, let’s say we want to find the derivative of h(x). We might be tempted to find the derivative of f(x) and g(x) separately and then multiply them together to get the derivative of h(x). However, this approach is not always correct because it doesn’t take into account the fact that g(x) is being used as the input for f(x). To account for this, we use the chain rule, which states that the derivative of a composite function can be found by multiplying the derivative of the outer function by the derivative of the inner function evaluated at the input.
In mathematical notation, the chain rule can be expressed as follows:
If h(x) = f(g(x)), then h'(x) = f'(g(x)) * g'(x)
Let’s look at an example to see how the chain rule works. Suppose we have the function h(x) = cos(x^2). We can see that h(x) is made up of two functions: f(x) = cos(x) and g(x) = x^2. The outer function f(x) is applied to the inner function g(x), so we can use the chain rule to find the derivative of h(x):
h'(x) = -sin(x^2) * 2x
Here, we first find the derivative of f(x) = cos(x), which is -sin(x). Then, we evaluate the derivative of g(x) = x^2 at the input x to get g'(x) = 2x. Finally, we multiply the two derivatives together to get the derivative of h(x).
The chain rule is particularly useful in cases where the inner function is not a simple polynomial, but rather a more complex function. For example, if we have the function h(x) = sin(cos(x)), we can see that the inner function g(x) = cos(x) is being used as input for the outer function f(x) = sin(x). We can use the chain rule to find the derivative of h(x):
h'(x) = cos(cos(x)) * -sin(x)
Here, we first find the derivative of f(x) = sin(x), which is -cos(x). Then, we evaluate the derivative of g(x) = cos(x) at the input x to get g'(x) = -sin(x). Finally, we multiply the two derivatives together to get the derivative of h(x).
In conclusion, the chain rule is a powerful tool that allows us to find derivatives of composite functions. It enables us to break down complex functions into simpler parts and find their derivatives using the derivative rules we already know. The chain rule is particularly useful in cases where the inner function is not a simple polynomial but a more complex function. Understanding the chain rule and how to apply it is essential for anyone studying calculus, as it is a fundamental concept that is used extensively in higher-level math and beyond.