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IB DP Maths AA SL Study Notes

5.1.3 Chain Rule

Introduction

In the vast landscape of maths, we frequently come across functions that are composed of other functions. These are termed as 'composite functions'. Differentiating such functions directly can often be a daunting task. However, the Chain Rule provides a systematic method to differentiate these composite functions, making the process more streamlined and efficient.

The Formula

At its core, the Chain Rule is a simple yet profound concept. Suppose you have a function y = f(u) and another function u = g(x). The Chain Rule states that the derivative of y with respect to x, represented as dy/dx, is:

dy/dx = f'(u) * g'(x)

In layman's terms, to differentiate a composite function, you first differentiate the outer function while keeping the inner function unchanged. Then, multiply the result by the derivative of the inner function. For a deeper understanding, you might want to review the Product and Quotient Rules.

Delving Deeper: Why the Chain Rule Works

Imagine a scenario where you're observing how a change in one quantity affects another through a chain of intermediate steps. The Chain Rule essentially captures this idea. It quantifies how a small change in the input (x) affects the intermediate function (u = g(x)), and then how this change in u affects the output (y = f(u)). By multiplying these rates of change together, we get the overall rate of change of y with respect to x.

Applications of the Chain Rule

1. Differentiating Exponential Functions with Bases Other than 'e'

Exponential functions with bases other than the natural number 'e' can be tricky. For instance, consider y = 2x. Using the Chain Rule, its derivative becomes:

dy/dx = ln(2) * 2x

This result is fascinating because it shows that the rate of change of the function is proportional to the function itself, with the proportionality constant being the natural logarithm of the base. To refresh your knowledge on related topics, visit Exponential Equations.

2. Differentiating Trigonometric Functions of Linear Functions

Trigonometric functions often have linear functions as their arguments. For example, y = sin(3x + 2). The Chain Rule simplifies its differentiation:

dy/dx = cos(3x + 2) * 3

This result indicates that the rate of change of the sine function is governed by the cosine of the same argument, scaled by the coefficient of x in the argument. You might find it helpful to review Graphs of Sine functions for more context.

3. Differentiating Nested Functions

Functions can sometimes be nested within one another, like y = (x2 + 1)3. The Chain Rule is invaluable here:

dy/dx = 3(x2 + 1)2 * 2x

This differentiation showcases the power of the Chain Rule in handling functions that have multiple layers of composition.

4. Differentiating Inverse Functions

The Chain Rule is also instrumental when differentiating inverse functions. For instance, if y = f(-1)(x) is the inverse of the function y = f(x), then:

dy/dx = 1 / f'(y)

This result is particularly useful when dealing with functions like arcsin, arccos, and arctan, which are inverses of the sine, cosine, and tangent functions, respectively. For more detailed explanations, see the notes on Finding Inverse Functions.

5. Differentiating Logarithmic Functions of Other Functions

Consider the function y = ln(x2 + 1). The Chain Rule aids in its differentiation:

dy/dx = (2x) / (x2 + 1)

This result highlights the Chain Rule's versatility in handling a wide range of functions, from exponential and trigonometric to logarithmic. Understanding Logarithmic Equations can also provide additional insights.

Example Questions

1. Differentiate the function y = sqrt(4x3 - x).

Using the Chain Rule, we first rewrite the function as y = (4x3 - x)(1/2). Differentiating:

dy/dx = (1/2)(4x3 - x)(-1/2) * (12x2 - 1)

2. Differentiate the function y = cos(x2 + 5x - 3).

Applying the Chain Rule:

dy/dx = -sin(x2 + 5x - 3) * (2x + 5)

FAQ

Yes, a graphical interpretation of the Chain Rule can be visualised using function graphs. Imagine plotting the graph of the inner function and then "feeding" this graph into the outer function. The slope of the tangent to the resulting composite function graph at any point gives the derivative. The Chain Rule essentially multiplies the rate of change (slope) of the inner function by the rate of change of the outer function. This visual approach can provide an intuitive grasp of why the Chain Rule works.

Absolutely! While the examples provided focused on trigonometric functions, the Chain Rule is a general differentiation technique that can be applied to any composite function. This includes exponential functions, logarithmic functions, polynomial functions, and more. Whenever one function is nested inside another, the Chain Rule can be employed to find the derivative of the composite function.

The Chain Rule is aptly named because it deals with functions that are composed of two or more other functions, much like links in a chain. When you have a composition of functions, the output of one function becomes the input of another. The Chain Rule provides a method to differentiate such composite functions. It allows us to "break apart" the chain and differentiate each link (function) separately, then combine the results to get the derivative of the entire composite function.

The Chain Rule is fundamental in various real-world scenarios, especially in physics and engineering. For instance, when considering how a change in one quantity might affect another quantity that is indirectly related, the Chain Rule provides the mathematical framework to understand these relationships. Examples include understanding how changing the angle of incidence affects the path of a ray of light through a lens or how a change in temperature might affect the rate of a chemical reaction.

One common mistake is forgetting to differentiate the inner function. Students often differentiate the outer function correctly but neglect the inner function's derivative. Another error is misapplying the rule when there are multiple terms; the Chain Rule should be applied to each term separately. It's also essential to be cautious with negative signs and powers. Regular practice and careful attention to each step can help avoid these pitfalls.

Practice Questions

Differentiate the function f(x) = sin(x^3 + 4x) using the chain rule.

To differentiate the function f(x) = sin(x3 + 4x) using the chain rule, we first identify the outer function as sin(u) and the inner function as u = x3 + 4x. Differentiating the outer function with respect to u, we get cos(u). Differentiating the inner function with respect to x, we get 3x2 + 4. Now, using the chain rule, the derivative is the product of these two derivatives. Therefore, the derivative of f(x) is (3x2 + 4) cos(x3 + 4x).

Differentiate the function g(x) = (x^2 + 3x + 2) / (x^3 - x) using the quotient rule.

To differentiate the function g(x) = (x2 + 3x + 2) / (x3 - x) using the quotient rule, we use the formula: d/dx (u/v) = (u'v - uv') / v2 Where u = x2 + 3x + 2 and v = x3 - x. Differentiating u with respect to x, we get 2x + 3. Differentiating v with respect to x, we get 3x2 - 1. Plugging these values into the formula, we get: (2x + 3)(x3 - x) - (x2 + 3x + 2)(3x2 - 1) / (x3 - x)2 Simplifying this expression, we get the derivative of g(x) as: -x2 - 4x + 2 / (x3 - x)2.

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
LinkedIn
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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