# Composite functions

##### Intros
###### Lessons
1. What is a "Composite Function"?
2. Quick review on basic operations with functions.
3. Composition of Functions: putting one function inside another function!
##### Examples
###### Lessons
1. Introduction to Composite Functions
If $f(x)= 4x-5$
$g(x)=8x^2+6$
Determine
1. $(f\circ g)(x)$
2. $(g\circ f)(x)$
3. $(f\circ f)(x)$
4. $(g\circ g)(x)$
5. $(f\circ g)(2)$
evaluate in two different ways
2. Evaluate a Composite Function Graphically
Use the graphs of $f(x)$ and $g(x)$ to evaluate the following:

1. $f(g(-4))$
2. $f(g(0))$
3. $g(f(-2))$
4. $g(f(-3))$
3. Determine the Composition of Three Functions
Use the functions $f(x)=3x,$
$g(x)=x-7$
and $h(x)=x^2$
to determine each of the following:
1. $(f\circ g\circ h)(x)$
2. $g(f(h(x)))$
3. $f(h(g(x)))$
4. $(h\circ g\circ f)(x)$
4. Determine the Original Functions From a Composite Function
If $h(x)=(f\circ g)(x)$
determine $f(x)$
and $g(x)$
1. $h(x)=(7x-5)^3-4(7x-5)+1$
2. $h(x)=\sqrt{4x^3-9}$, give two possible sets of solutions
5. Composite Functions with Restrictions
Consider $f(x)=\sqrt{x-13}$
and $g(x)=x^2+4$, for each of the function below:
1. Determine:
i)
$(f\circ g)(x)$
ii)
$(g\circ f)(x)$
2. State the domain and range of:
i)
$f(x)$
ii)
$g(x)$
iii) $(f\circ g)(x)$
iv)
$(g\circ f)(x)$
3. Sketch the graph of:
i)
$f(x)$
ii)
$g(x)$
iii) $(f\circ g)(x)$
iv)
$(g\circ f)(x)$
6. Composite Functions with Restrictions
Consider $f(x)=\sqrt{x-13}$
and $g(x)=x^2+4$, for each of the function below:
i) state the domain and range
ii) sketch the graph.
1. $f(x)$
2. $g(x)$
3. $(f\circ g)(x)$

## Introduction to Composite Functions

Composite functions are a fundamental concept in mathematics that allows us to combine two or more functions to create a new function. The introduction video provides a crucial foundation for understanding this concept, making it an essential starting point for students. By watching this video, learners gain insight into how composite functions work and their significance in various mathematical applications. Composite functions are formed by applying one function to the output of another, creating a chain of operations. This process is denoted using specific notation and can be evaluated step-by-step. Understanding composite functions is vital for solving complex mathematical problems and is widely used in fields such as physics, engineering, and computer science. The ability to evaluate composite functions opens up new possibilities for analyzing and modeling real-world scenarios. As students progress in their mathematical journey, mastering composite functions becomes increasingly important, serving as a building block for more advanced topics in calculus and beyond.

## Understanding Composite Functions

Composite functions are a fundamental concept in mathematics that involves combining functions to create a new function. To understand this concept better, let's use a simple analogy: imagine putting an apple inside a lunch bag. In this scenario, the apple represents one function, and the lunch bag represents another function. When we put the apple inside the lunch bag, we're essentially creating a composite function.

In mathematical terms, a composite function is formed when the output of one function becomes the input of another function. This process is similar to how the apple (one function) becomes contained within the lunch bag (another function). The resulting composite function represents a new, more complex operation that combines the effects of both original functions.

The function notation used to represent composite functions is f(g(x)), where f and g are two different functions. To read this notation, we start from the innermost function and work our way outward. In our example, g(x) would represent the apple, and f would represent the lunch bag. So, f(g(x)) means we first apply function g to x (put the apple in), and then apply function f to the result (close the lunch bag).

It's crucial to emphasize that composite functions involve putting one function inside another function. This concept is not just about performing two separate operations in sequence; it's about nesting one function within another. In our analogy, we're not simply placing an apple next to a lunch bag; we're putting the apple inside the bag, creating a new entity (the packed lunch) that combines properties of both the apple and the bag.

When working with composite functions, the order of operations in functions is significant. Just as putting an apple in a lunch bag is different from putting a lunch bag inside an apple, f(g(x)) is not the same as g(f(x)). The inner function (g in this case) is always applied first, and its output becomes the input for the outer function (f).

Understanding composite functions is essential in various fields of mathematics and its applications. They allow us to model complex relationships and processes that involve multiple stages or transformations. For instance, in physics, we might use composite functions to describe the motion of an object affected by multiple forces, or in economics, to model the impact of various factors on market prices.

To further illustrate the concept, let's consider a numerical example. Suppose we have two functions: f(x) = x² and g(x) = x + 3. The composite function f(g(x)) would be calculated as follows: first, we apply g(x), which gives us x + 3. Then, we take this result and apply f to it. So, f(g(x)) = f(x + 3) = (x + 3)². This new function combines the effects of both original functions in a specific order.

Practicing with composite functions helps develop a deeper understanding of function relationships and enhances problem-solving skills in mathematics. It's a powerful tool that allows us to break down complex problems into simpler components and then combine them in meaningful ways. As you work with composite functions, remember the apple and lunch bag analogy it's all about putting one function inside another to create a new, more intricate mathematical structure.

## Strategy for Evaluating Composite Functions

Evaluating composite functions can seem daunting at first, but with the right strategy, it becomes a manageable and even intuitive process. The key to mastering how to evaluate composite functions lies in understanding the "outside-in" approach. This method is not only effective but also aligns with how composite functions are structured.

To begin, let's clarify what we mean by "outside" and "inside" functions in a composite function. Imagine a lunch bag containing an apple. The lunch bag represents the outside function, while the apple inside represents the inside function. Just as you need to open the lunch bag to access the apple, you need to work through the outside function to evaluate the inside function in a composite function.

Identifying the outside and inside functions is crucial. The outside function is the one that "wraps" around the entire expression, while the inside function is the argument of the outside function. For example, in f(g(x)), f is the outside function, and g(x) is the inside function.

Here's a step-by-step guide on how to evaluate composite functions using the outside-in strategy:

1. Identify the outside function and the inside function.
2. Evaluate the inside function first, treating it as a separate entity.
3. Take the result from step 2 and use it as the input for the outside function.
4. Evaluate the outside function with this new input.

Let's reinforce this concept with our lunch bag and apple analogy. Imagine you want to eat the apple (evaluate the function). You can't directly access the apple; you must first open the lunch bag (work through the outside function). Once the bag is open, you can reach in and grab the apple (evaluate the inside function). Finally, you can enjoy the apple (obtain the final result).

This analogy helps visualize the process: just as you can't eat the apple without first opening the bag, you can't evaluate the inside function without first considering how it fits into the outside function.

When setting up the evaluation process, it's helpful to write out each step clearly:

1. Write the composite function: f(g(x))
2. Identify f as the outside function and g(x) as the inside function
3. Evaluate g(x) first, replacing x with the given value
4. Write the result as f(result from step 3)
5. Now evaluate this new function f

Remember, the key to successfully evaluating composite functions is to work methodically from the outside in. This approach ensures you don't miss any steps and helps prevent common errors. With practice, you'll find that this strategy becomes second nature, allowing you to tackle even complex composite functions with confidence.

As you become more comfortable with how to evaluate composite functions, you'll notice patterns and develop a intuition for the process. This skill is invaluable in higher-level mathematics and real-world applications where functions often combine in complex ways. By mastering the outside-in strategy, you're not just learning to solve specific problems; you're developing a fundamental understanding of how functions interact and build upon each other.

## Worked Example: f(g(x))

Let's dive into a detailed worked example of evaluating a composite function, specifically f(g(x)), using the functions f(x) = 2x - 5 and g(x) = 4x - 3. This step-by-step evaluation process will demonstrate the crucial concept of function composition and highlight the importance of replacing x with the inside function.

Step 1: Identify the functions
We have two functions:
f(x) = 2x - 5
g(x) = 4x - 3

Step 2: Understand the notation
f(g(x)) means we need to evaluate f(x) where x is replaced by g(x). In other words, we'll be plugging the entire g(x) function into f(x) wherever we see x.

Step 3: Write out f(g(x))
f(g(x)) = 2(g(x)) - 5
Notice that we've replaced x in f(x) with g(x).

Step 4: Substitute g(x)
Now, we replace g(x) with its actual expression:
f(g(x)) = 2(4x - 3) - 5

Step 5: Simplify the expression
Let's work through this step-by-step:
a) First, distribute the 2:
f(g(x)) = 2(4x) - 2(3) - 5
b) Simplify:
f(g(x)) = 8x - 6 - 5

Step 6: Combine like terms
f(g(x)) = 8x - 11

And there we have it! We've successfully evaluated f(g(x)) and arrived at our final expression: 8x - 11.

This worked example demonstrates several key points in evaluating composite functions:

1. Order matters: We started with the inside function g(x) and then applied f(x) to it. This order is crucial in function composition.

2. Substitution: The critical step was replacing x in f(x) with the entire expression of g(x). This substitution is the essence of function composition.

3. Algebraic manipulation: After substitution, we used standard algebraic techniques to simplify the expression.

4. Final form: The result is a new function that takes x as input and produces a different output than either f(x) or g(x) alone.

Understanding this process is fundamental to working with more complex composite functions and is a crucial skill in advanced mathematics, including calculus and beyond. It's important to practice this technique with various functions to build proficiency and intuition.

Remember, when evaluating f(g(x)), always work from the inside out. Start with g(x), then apply f to that result. This methodical approach will help you avoid common mistakes and successfully navigate more complex composite functions.

In real-world applications of composite functions, composite functions appear in many areas, such as physics (where one quantity depends on another, which in turn depends on a third), economics (where complex relationships between variables are modeled), and computer science (in algorithm design and function chaining).

By mastering this concept, you're equipping yourself with a powerful tool for mathematical modeling and problem-solving across various disciplines. Continue to practice with different functions and more complex compositions to further strengthen your understanding and skills in this essential mathematical concept. Real-world applications of composite functions are vast and varied, making this a valuable skill to develop.

## Order Matters: Comparing f(g(x)) and g(f(x))

In the world of mathematics, particularly in the realm of functions, the order of composition plays a crucial role. This concept is beautifully illustrated when we compare the outcomes of f(g(x)) and g(f(x)) using the same functions. Let's delve into this fascinating aspect of composite functions and understand why the order is so important.

To demonstrate this, let's revisit our previous functions: f(x) = x² + 1 and g(x) = 2x - 3. We've already explored f(g(x)), so now let's evaluate g(f(x)) and compare the results.

For g(f(x)), we first apply function f to x, and then apply function g to the result. Here's how it works:

g(f(x)) = g(x² + 1)
= 2(x² + 1) - 3
= 2x² + 2 - 3
= 2x² - 1

Now, let's recall our result for f(g(x)):

f(g(x)) = f(2x - 3)
= (2x - 3)² + 1
= 4x² - 12x + 9 + 1
= 4x² - 12x + 10

Comparing these results, we can clearly see that g(f(x)) = 2x² - 1 is different from f(g(x)) = 4x² - 12x + 10. This difference underscores the importance of the order in composite functions.

But why does the order matter so much? The key lies in the nature of function composition itself. When we compose functions, we're essentially nesting one function inside another. The inner function is applied first, and its output becomes the input for the outer function. This sequence of operations can lead to dramatically different results depending on which function is applied first.

In our example, when we calculate f(g(x)), we first subtract 3 from 2x, and then square the result before adding 1. On the other hand, when we calculate g(f(x)), we first square x and add 1, and then double this result before subtracting 3. These different sequences of operations naturally lead to different outcomes.

This principle extends beyond simple algebraic functions. In more complex scenarios, such as in calculus or in practical applications of functions like computer graphics or signal processing, the order of function composition can have significant implications. For instance, in 3D graphics, the order in which you apply transformations (like rotation, scaling, and translation) to an object can result in entirely different final positions and orientations.

Understanding the non-commutativity of function composition (that is, f(g(x)) g(f(x)) in general) is crucial in many areas of mathematics and its applications. It reminds us to be careful and precise when working with composite functions, always paying attention to the order in which operations are performed.

In conclusion, the order of composition in functions is not just a mathematical curiosity; it's a fundamental concept with far-reaching implications. By comparing f(g(x)) and g(f(x)), we've seen how the same set of functions can produce entirely different results when composed in different orders. This principle serves as a powerful reminder of the importance of sequence and structure in mathematical operations, encouraging us to approach problem-solving with careful consideration of the order in which we apply our mathematical tools in practical applications of functions.

## Applications and Practice Problems

Composite functions applications play a crucial role in various real-world applications, demonstrating their relevance across multiple fields. Understanding these applications and practicing problem-solving techniques can significantly enhance one's grasp of composite functions applications.

### Real-World Applications

1. Economics: Composite functions applications are used to model complex economic relationships, such as how changes in interest rates affect consumer spending and, in turn, impact GDP.

2. Physics: In kinematics, composite functions describe the position of an object as a function of time, which itself may be a function of initial velocity and acceleration.

3. Computer Graphics: Transformations in computer graphics often involve composing multiple functions to achieve complex visual effects.

4. Biology: Growth models in population dynamics frequently use composite functions to represent the interplay between various factors affecting population size.

### Practice Problems

To reinforce your understanding of composite functions practice problems, try solving the following problems:

#### Beginner Level

1. Given f(x) = x² + 3 and g(x) = 2x - 1, find (f g)(2).

Hint: First calculate g(2), then use that result as the input for f.

2. If h(x) = x and k(x) = x + 5, determine (h k)(4).

Hint: Start by finding k(4), then apply h to that result.

#### Intermediate Level

3. Let f(x) = 1/(x-2) and g(x) = x² - 1. Find the domain of (f g)(x).

Hint: Consider the restrictions on both f and g, and how they interact when composed.

4. Given f(x) = ln(x) and g(x) = e^x, prove that (f g)(x) = x for all real numbers x.

Hint: Apply the definition of composite functions and use the properties of logarithms and exponentials.

5. Let f(x) = x³ - 3x and g(x) = x² + 2. Find a formula for (f g)(x) using the chain rule.

Hint: Remember the chain rule states (f g)(x) = f(g(x)) · g(x).

6. Given f(x) = sin(x) and g(x) = arcsin(x), show that (f g)(x) = x for -1 x 1, but (g f)(x) x for all x.

Hint: Consider the domains and ranges of both functions and how they interact when composed.

### Step-by-Step Solutions

1. (f g)(2) = f(g(2)) g(2) = 2(2) - 1 = 3 f(3) = 3² + 3 = 12 Therefore, (f g)(2) = 12

2. (h k)(4) = h(k(4)) k(4) = 4 + 5 = 9 h(9) = 9 = 3 Therefore, (h k)(4) = 3

3. Domain of (f g)(x): g(x) = x² - 1 is defined for all real numbers.

## Conclusion

Composite functions are a crucial concept in mathematics, combining two or more functions to create a new one. The key points to remember include the importance of order when composing functions, as f(g(x)) is not always equal to g(f(x)). When evaluating composite functions, work from the inside out, starting with the innermost function. The introduction video provides a solid foundation for understanding these concepts, making it an essential resource for learners. To truly grasp composite functions, regular practice is vital. Start with simple examples and gradually progress to more complex ones. This approach will help reinforce your understanding and improve your problem-solving skills. As you become more comfortable with basic composite functions, explore more advanced applications in various fields such as physics, economics, and computer science. Remember, mastering composite functions opens doors to understanding more sophisticated mathematical concepts and real-world applications. Keep practicing and expanding your knowledge to unlock new possibilities in mathematics and beyond.

### Example:

Introduction to Composite Functions
If $f(x)= 4x-5$

$g(x)=8x^2+6$
Determine
$(f\circ g)(x)$

#### Step 1: Understand the Concept of Composite Functions

Composite functions involve applying one function to the results of another function. In mathematical notation, this is often written as $(f \circ g)(x)$, which means $f(g(x))$. The function $g(x)$ is applied first, and then the result of $g(x)$ is used as the input for the function $f(x)$. This process is often referred to as working from the "inside out" or "outside in".

#### Step 2: Identify the Outer and Inner Functions

In the given problem, we have two functions:

• $f(x) = 4x - 5$
• $g(x) = 8x^2 + 6$
To find $(f \circ g)(x)$, we need to identify which function is the outer function and which is the inner function. Here, $f(x)$ is the outer function and $g(x)$ is the inner function. This means we will first apply $g(x)$ and then apply $f(x)$ to the result of $g(x)$.

#### Step 3: Write Down the Outer Function

Start by writing down the outer function $f(x)$. In this case, $f(x) = 4x - 5$. When dealing with composite functions, it is helpful to write the function in a way that makes it clear where the inner function will be substituted. So, we can write: $f(x) = 4(x) - 5$ This notation emphasizes that whatever is inside the parentheses will be multiplied by 4 and then 5 will be subtracted.

#### Step 4: Substitute the Inner Function into the Outer Function

Next, we substitute the inner function $g(x)$ into the outer function $f(x)$. This means replacing every instance of $x$ in $f(x)$ with $g(x)$. So, we have: $f(g(x)) = 4(g(x)) - 5$ Given that $g(x) = 8x^2 + 6$, we substitute $g(x)$ into the equation: $f(g(x)) = 4(8x^2 + 6) - 5$

#### Step 5: Simplify the Expression

Now, we need to simplify the expression by performing the arithmetic operations. First, distribute the 4 through the parentheses: $4(8x^2 + 6) = 4 \cdot 8x^2 + 4 \cdot 6 = 32x^2 + 24$ Then, subtract 5 from the result: $32x^2 + 24 - 5$ Finally, combine like terms to simplify the expression: $32x^2 + 19$

#### Step 6: Write the Final Composite Function

After simplifying, we have the final form of the composite function: $(f \circ g)(x) = 32x^2 + 19$ This is the result of applying the function $f$ to the function $g$.

### FAQs

#### 1. How do you solve for composite functions?

To solve composite functions, follow these steps:

1. Identify the inner and outer functions.
2. Substitute the inner function into the outer function wherever you see the variable.
3. Simplify the resulting expression.
4. If a specific value is given, evaluate the simplified expression using that value.

#### 2. How do you prove a function is composite?

To prove a function is composite, you need to show that it can be expressed as a combination of two or more simpler functions. This can be done by identifying the component functions and demonstrating how they combine to form the given function.

#### 3. What is an example of a composite function?

A common example of a composite function is f(g(x)) = (2x - 3)² + 1, where f(x) = x² + 1 and g(x) = 2x - 3. Here, g(x) is applied first, and then its result becomes the input for f(x).

#### 4. How to identify composite functions?

Composite functions can be identified by looking for nested operations or functions within functions. If you can separate a function into two or more distinct operations that are applied sequentially, it's likely a composite function.

#### 5. What is the rule for composite functions?

The rule for composite functions is (f g)(x) = f(g(x)). This means you apply the inner function g(x) first, then use its output as the input for the outer function f(x). Remember that the order matters, as f(g(x)) is generally not equal to g(f(x)).

### Prerequisite Topics for Understanding Composite Functions

Mastering composite functions requires a solid foundation in several key mathematical concepts. One of the most crucial prerequisites is function notation. Understanding how to read and write functions using proper notation is essential when working with composite functions, as it allows you to clearly express the combination of two or more functions.

Another vital concept to grasp is the order of operations (PEMDAS). This fundamental principle becomes especially important when evaluating composite functions, as you'll need to perform operations in the correct sequence to obtain accurate results.

Building on these basics, combining transformations of functions is a topic that directly relates to composite functions. This skill helps you understand how different functions interact and how their transformations affect the final composite function.

While it may not seem immediately obvious, knowledge of distance and time related questions in linear equations can be beneficial when working with composite functions. Many real-world applications of composite functions involve scenarios where multiple variables, such as distance and time, are interrelated.

Similarly, understanding the applications of linear relations provides a practical context for composite functions. This knowledge helps you recognize situations where composite functions can be applied to solve complex problems involving multiple linear relationships.

Lastly, although it's a more advanced topic, familiarity with the chain rule in calculus is incredibly valuable when studying composite functions. The chain rule is essentially the method for differentiating composite functions, making it an indispensable tool for those pursuing higher-level mathematics.

By mastering these prerequisite topics, you'll be well-equipped to tackle the complexities of composite functions. Each concept builds upon the others, creating a strong foundation for understanding how functions can be combined and manipulated. Function notation gives you the language to express composite functions, while the order of operations ensures you evaluate them correctly. Combining transformations helps you visualize how composite functions behave, and real-world applications like distance-time problems showcase their practical uses.

Moreover, understanding linear relations and their applications provides context for when and why composite functions are used in various fields. And for those continuing into calculus, the chain rule becomes a powerful tool for analyzing composite functions at an even deeper level. By investing time in these prerequisite topics, you'll not only enhance your ability to work with composite functions but also develop a more comprehensive understanding of mathematics as a whole.