# Inequalities of combined functions

##### Intros
###### Lessons
1. Introduction to inequalities of combined functions

i. What are inequalities of combined functions?

ii. How many ways can it be solved?

##### Examples
###### Lessons
1. Evaluating Inequalities of Combined Functions by Comparing the Functions Graphically

Let $f(x) = 2x^{2}$ and $g(x) = 3x + 2$.

1. Graph the functions on the same set of axes. Identify the points of intersection.
2. Illustrate the regions for which

i. $f(x)$ > $g(x)$

ii. $g(x)$ > $f(x)$

2. Evaluating Inequalities of Combined Functions by Analyzing the Difference Function

Let $f(x) = 2x^{2} + x - 3$ and $g(x) = x^{2} + x + 13$.

1. Graph the difference function.
2. Illustrate the regions for which

i. $f(x)$ > $g(x)$

ii. $g(x)$ > $f(x)$

3. Let $f(x) = (x-3)(x+5)$ and $g(x) = (x+1)(x-4)$
1. Graph the difference function.
2. Illustrate the regions for which

i. $f(x)$ > $g(x)$

ii. $g(x)$ > $f(x)$

4. Evaluating Inequalities of Combined Functions by Analyzing the Quotient Function

Let $f(x) = (x+3)^{6}$ and $g(x) = (x+3)^{4}$

1. Graph the quotient function.
2. Illustrate the regions for which

i. $f(x)$ > $g(x)$

ii. $g(x)$ > $f(x)$

5. Application of Inequalities of Combined Functions

Nick is starting his own phone company. The cost of making and storing phones can be modelled by the function:

$C(n) = 1.2n + \frac{150,000}{n}$

where $n$ = number of phones. The storage capacity of the company's warehouse is 500 units.

1. Use graphing technology to graph C($n$). What is the domain of this function?
2. Determine the number of phones that can be made if Nick wants to keep the cost below \$1000.

## Introduction

Inequalities of combined functions present a challenging yet essential topic in advanced mathematics. Our introduction video serves as a crucial starting point, offering a visual and conceptual foundation for understanding these complex mathematical relationships. This video demystifies the core concepts, preparing you for a deeper exploration of the subject. In this article, we'll delve into three primary approaches to solving inequalities involving combined functions. These methods will equip you with the tools to tackle a wide range of problems, from basic to advanced levels. By mastering these techniques, you'll enhance your problem-solving skills and gain a more profound understanding of functional relationships. Whether you're a student aiming to excel in mathematics or an enthusiast looking to broaden your knowledge, this comprehensive guide will illuminate the intricacies of inequalities with combined functions, building upon the insights provided in our introductory video.

Our introduction video serves as a crucial starting point, offering a visual and conceptual foundation for understanding these complex mathematical relationships. This video demystifies the core concepts, preparing you for a deeper exploration of the subject. In this article, we'll delve into three primary approaches to solving inequalities involving combined functions. These methods will equip you with the tools to tackle a wide range of problems, from basic to advanced levels.

## Comparing Functions Graphically

When it comes to solving inequalities involving combined functions, one of the most intuitive approaches is graphical comparison. This method allows us to visualize the relationship between two functions and determine where one is greater than the other. Let's dive into this approach and explore how it works.

The process of graphical comparison involves two main steps. First, we graph both functions on the same set of axes in the xy plane. This gives us a visual representation of how the functions behave relative to each other. Second, we identify the regions where one function is greater than the other by examining where the graphs intersect and how they're positioned.

To illustrate this method, let's consider an example with a linear function f(x) and a quadratic function g(x). Suppose we want to solve the inequality f(x) > g(x), where f(x) = 2x + 1 and g(x) = x² - 3x + 1. Here's how we'd approach this using graphical comparison:

Step 1: Graph both functions on the same coordinate system. We'd plot the linear function f(x) = 2x + 1, which appears as a straight line, and the quadratic function g(x) = x² - 3x + 1, which forms a parabola.

Step 2: Identify the regions where f(x) > g(x). We look for areas where the line (representing f(x)) is above the parabola (representing g(x)). In this case, we'd see that the line intersects the parabola at two points. The region where f(x) is greater than g(x) would be between these two intersection points.

The beauty of this graphical approach is that it provides a clear visual understanding of the solution. We can easily see where one function "dominates" the other. In our example, the line would be above the parabola in the middle section of the graph, indicating that f(x) > g(x) in this interval.

It's important to note that while this method is highly intuitive, it may not always provide precise numerical solutions. For exact values, we might need to combine this approach with algebraic methods. However, the graphical comparison gives us a solid starting point and a conceptual grasp of the solution.

This technique is particularly useful when dealing with complex functions or when you want to quickly understand the general behavior of an inequality. It allows you to see patterns and relationships that might not be immediately apparent from the equations alone.

Remember, when using this method, pay close attention to the points where the graphs intersect. These intersection points are crucial as they often represent the boundaries of the solution regions. In our example with the line and parabola, the x-coordinates of the intersection points would give us the exact range where f(x) > g(x).

Graphical comparison is a powerful tool in your mathematical toolkit. It bridges the gap between abstract equations and visual understanding, making complex inequalities more accessible. As you practice this method, you'll find that it enhances your overall comprehension of function behavior and relationships.

In conclusion, the graphical comparison approach to solving inequalities of combined functions is a valuable skill. It involves graphing both functions on the same xy plane and identifying regions where one function is greater than the other. This method provides a visual and intuitive understanding of the solution, making it an excellent starting point for analyzing complex mathematical relationships.

## Analyzing the Difference Function

When solving inequalities involving combined functions, the difference function emerges as a powerful second approach. This method provides a clear and intuitive way to determine where one function is greater than another. The difference function is defined as f(x) - g(x), where f(x) and g(x) are the two functions being compared.

The concept of the difference function is straightforward: it subtracts one function from another, creating a new function that represents the vertical distance between functions at any given point. By analyzing where this new function is positive, negative, or zero, we can gain valuable insights into the relationship between f(x) and g(x).

When the difference function f(x) - g(x) is positive, it indicates that f(x) > g(x). This means that the graph of f(x) lies above the graph of g(x) for those x-values. Conversely, when f(x) - g(x) is negative, it signifies that f(x) < g(x), implying that the graph of f(x) is below that of g(x). The points where f(x) - g(x) equals zero are particularly significant, as they represent the intersections of the two original functions.

To illustrate this concept, let's consider an example. Suppose we want to solve the inequality f(x) > g(x), where f(x) = x² + 2 and g(x) = 2x + 1. We can create the difference function h(x) = f(x) - g(x) = x² - 2x + 1. By analyzing h(x), we can determine where f(x) > g(x) is true. The solution to our inequality will be the x-values where h(x) > 0.

The significance of the difference function extends beyond solving inequalities. It provides a visual and algebraic tool for comparing functions, identifying intervals of dominance, and understanding the behavior of combined functions. In calculus, the difference function plays a crucial role in optimization problems, helping to find maximum and minimum values in various applications.

Moreover, the concept of the difference function can be applied to more complex scenarios involving multiple functions or higher-dimensional problems. It serves as a foundation for advanced mathematical analysis and problem-solving techniques in fields such as physics, engineering, and economics.

In conclusion, the difference function f(x) - g(x) offers a robust method for solving inequalities and analyzing the relationship between functions. By focusing on where this function is positive, negative, or zero, we can gain deep insights into function behavior and solve a wide range of mathematical problems efficiently and effectively.

## Analyzing the Quotient Function

The third approach to determining inequalities between two functions involves analyzing the quotient function. This method provides a powerful tool for comparing functions and understanding their relative behavior. The quotient function is defined as f(x) / g(x), where f(x) and g(x) are the two functions being compared.

By examining the quotient function, we can gain valuable insights into the relationship between f(x) and g(x). The key principle is that the value of the quotient function relative to 1 determines which function is greater. Here's how it works:

• If f(x) / g(x) > 1, then f(x) > g(x)
• If f(x) / g(x) < 1, then f(x) < g(x)
• If f(x) / g(x) = 1, then f(x) = g(x)

Let's consider an example to illustrate this concept. Suppose we want to compare f(x) = x^2 and g(x) = 2x + 1. We can form the quotient function:

h(x) = f(x) / g(x) = x^2 / (2x + 1)

By analyzing h(x), we can determine when f(x) is greater than, less than, or equal to g(x). For instance, if we find that h(x) > 1 for x > 2, we can conclude that x^2 > 2x + 1 for all x > 2.

The significance of the quotient being greater than, less than, or equal to 1 lies in its ability to provide a clear threshold for comparison. When the quotient exceeds 1, the numerator (f(x)) is larger than the denominator (g(x)). Conversely, when it's less than 1, the denominator is larger. This simple relationship allows us to easily determine which function is greater for different ranges of x.

However, it's crucial to address potential issues with division by zero when using the quotient function approach. Division by zero occurs when g(x) = 0 for certain values of x. These points need special consideration as the quotient function is undefined at these values. To handle this, we must:

1. Identify all x-values where g(x) = 0
2. Exclude these points from our analysis
3. Examine the behavior of f(x) and g(x) near these points separately

For example, if g(x) = x - 2, we need to be cautious around x = 2. We would analyze the quotient function for x < 2 and x > 2 separately, and then consider the behavior of both functions as x approaches 2 from both sides.

In conclusion, the quotient function approach offers a systematic way to compare two functions. By examining f(x) / g(x) and its relationship to 1, we can determine inequalities between f(x) and g(x) across different domains. This method is particularly useful for complex functions where direct comparison might be challenging. However, it's essential to be mindful of potential division by zero issues and handle them appropriately to ensure a comprehensive and accurate analysis.

## Choosing the Right Approach

When solving optimization problems, it's crucial to select the most appropriate method among the graphical, difference function, and quotient function approaches. Each technique has its own strengths and limitations, and understanding when to use each one can significantly impact the efficiency and accuracy of your problem-solving process.

The graphical approach is often the first method considered due to its visual nature. It's particularly useful when dealing with simple functions or when you need a quick, intuitive understanding of the problem. This method works well for functions that are easy to graph and when the optimal points are visually apparent. For instance, when optimizing a quadratic function, the graphical approach can quickly reveal the maximum or minimum point. However, its limitations become evident with complex functions or when precise values are required, as visual estimations may lack accuracy.

The difference function method is particularly effective when dealing with problems involving the difference between two quantities. This approach shines when optimizing the difference between two functions, such as maximizing profit (revenue minus cost) or finding the optimal point where two changing quantities diverge the most. Its advantage lies in its ability to simplify complex problems by focusing on the relationship between two functions. However, it may not be the best choice for problems that don't naturally involve a difference or when the resulting function is difficult to analyze.

The quotient function approach is ideal for optimization problems involving ratios or rates. It's particularly useful in scenarios such as maximizing efficiency (output divided by input) or optimizing rates of change. This method excels in problems where the relationship between two quantities is best expressed as a fraction. Its strength lies in its ability to handle problems that naturally occur as ratios in fields like physics, economics, and engineering. However, the quotient function can sometimes lead to more complex calculations, especially when dealing with intricate functions.

When selecting the most appropriate approach, consider the nature of the problem and the given functions. If the problem involves simple functions and a visual understanding is sufficient, the graphical method may be your best bet. For problems centered around maximizing or minimizing the difference between two quantities, the difference function approach is often the most efficient. When dealing with rates, efficiencies, or any scenario where a ratio is central to the problem, the quotient function method is typically the most suitable.

Examples can help illustrate when each method might be preferable. For instance, when optimizing the area of a rectangle with a fixed perimeter, the graphical approach can provide a quick visual solution. In contrast, when maximizing the profit of a company by considering revenue and cost functions, the difference function method would be more appropriate. For problems like finding the optimal speed for fuel efficiency (distance traveled per unit of fuel), the quotient function approach would be the natural choice.

It's important to note that these methods are not mutually exclusive, and complex problems may benefit from a combination of approaches. Starting with a graphical approach to gain intuition, then moving to either the difference or quotient function method for precise calculations, can often lead to a comprehensive understanding and solution of the problem.

In conclusion, the key to effective problem-solving in optimization lies in understanding the strengths and limitations of each approach. By carefully analyzing the nature of the problem, the given functions, and the desired outcome, you can select the most appropriate method or combination of methods to arrive at an accurate and efficient solution. This strategic approach to problem-solving not only enhances your ability to tackle a wide range of optimization challenges but also deepens your understanding of the underlying mathematical principles at play.

## Practice Problems and Solutions

Let's dive into some practice problems involving inequalities of combined functions! We'll explore a variety of scenarios and solve them using different approaches. Remember, there's often more than one way to tackle these problems, so don't worry if your method looks a little different. The important thing is to understand the reasoning behind each step.

### Problem 1: Graphical Approach

Solve the inequality: f(x) + g(x) > 0, where f(x) = x² - 4 and g(x) = -x + 2

Solution:

1. First, let's combine the functions: f(x) + g(x) = (x² - 4) + (-x + 2) = x² - x - 2
2. Now, we need to graph y = x² - x - 2 and find where it's above the x-axis
3. The parabola opens upward and crosses the x-axis at x = -1 and x = 2
4. Therefore, the solution is x < -1 or x > 2

### Problem 2: Algebraic Approach

Solve the inequality: f(x) / g(x) 1, where f(x) = x + 3 and g(x) = x - 1

Solution:

1. Set up the inequality: (x + 3) / (x - 1) 1
2. Multiply both sides by (x - 1), being careful with the inequality sign:
• If x - 1 > 0: x + 3 x - 1
• If x - 1 < 0: x + 3 x - 1
3. Solve each case:
• Case 1 (x > 1): 4 0 (never true)
• Case 2 (x < 1): 4 0 (always true)
4. Don't forget x 1 (division by zero)
5. The solution is x < 1

### Problem 3: Properties of Inequalities

Solve the inequality: |f(x)| < g(x), where f(x) = 2x - 1 and g(x) = x + 3

Solution:

1. Recall that |a| < b is equivalent to -b < a < b
2. Apply this to our inequality: -(x + 3) < 2x - 1 < x + 3
3. Solve the left inequality: -x - 3 < 2x - 1
• Add x to both sides: -3 < 3x - 1
• Add 1 to both sides: -2 < 3x
• Divide by 3: -2/3 < x
4. Solve the right inequality: 2x - 1 < x + 3
• Subtract x from both sides: x - 1 < 3
• Add 1 to both sides: x < 4
5. Combine the results: -2/3 < x < 4

### Problem 4: Solving Quadratic Inequalities

Let's consider another example involving solving quadratic inequalities. Solve the inequality: x² - 5x + 6 < 0

Solution:

1. Factor the quadratic expression: (x - 2)(x - 3) < 0
2. Determine the critical points: x = 2 and x = 3
3. Test intervals around the critical points to find where the product is negative
4. The solution is 2 < x < 3

### Problem 5: Solving Rational Inequalities

Now, let's tackle a problem involving solving rational inequalities. Solve the inequality: (x + 1) / (x - 2) > 0

Solution:

1. Find the critical points: x = -1 and x = 2
2. Test intervals around the critical points to determine where the expression is positive
3. The solution is x < -1 or x > 2

## Real-world Applications

Inequalities of combined functions play a crucial role in various real-world applications, particularly in fields such as economics, physics, and engineering. Understanding and solving these types of inequalities is essential for modeling complex systems and making informed decisions in practical scenarios.

In economics, inequalities of combined functions are frequently used to analyze and optimize various aspects of business and finance. For instance, profit maximization problems often involve inequalities that combine revenue and cost functions. A company might need to determine the production level that maximizes profit while considering constraints on resources and market demand. This scenario can be modeled using inequalities that combine production, revenue, and cost functions, requiring the application of techniques discussed earlier in this article.

Another economic application is in portfolio optimization, where investors seek to maximize returns while minimizing risk. This involves inequalities that combine functions representing different investment options, expected returns, and risk factors. By solving these inequalities, financial analysts can determine the optimal allocation of assets in a portfolio that meets specific risk-return criteria.

In the field of physics, inequalities of combined functions are essential for describing and analyzing various phenomena. For example, in thermodynamics, the second law is often expressed as an inequality involving entropy and heat transfer. This fundamental principle governs the direction of natural processes and is crucial in understanding energy efficiency and designing thermal systems. Engineers working on heat engines or refrigeration cycles must solve inequalities that combine functions representing work, heat transfer, and efficiency to optimize system performance.

Another physics application is in optics, where inequalities are used to describe the behavior of light in different media. Snell's law, which governs the refraction of light, can be expressed as an inequality involving the angles of incidence and refraction, as well as the refractive indices of the media. This principle is fundamental in designing optical systems such as lenses, fiber optics, and imaging devices.

In engineering, inequalities of combined functions are ubiquitous in design and optimization problems. For instance, in structural engineering, the design of beams and columns involves inequalities that combine functions representing stress, strain, and material properties. Engineers must ensure that the stresses in a structure remain below certain thresholds while optimizing for factors such as weight and cost. This requires solving complex inequalities that incorporate multiple interrelated functions.

Another engineering application is in control systems, where inequalities are used to define stability criteria and performance bounds. For example, in designing a feedback control system for an aircraft, engineers must solve inequalities that combine functions representing the aircraft's dynamics, sensor measurements, and control inputs to ensure stable and robust performance across a range of operating conditions.

These real-world applications demonstrate the practical importance of understanding and solving inequalities of combined functions. The methods discussed earlier in this article, such as graphical approaches, algebraic techniques, and numerical methods, are directly applicable to these scenarios. By mastering these techniques, professionals in various fields can effectively analyze complex systems, optimize performance, and make informed decisions based on mathematical models that accurately represent real-world phenomena.

As technology advances and systems become increasingly complex, the ability to work with inequalities of combined functions becomes even more critical. From optimizing energy systems to designing advanced materials, these mathematical tools provide a powerful framework for addressing real-world challenges across diverse disciplines. By bridging the gap between abstract mathematical concepts and practical applications, inequalities of combined functions continue to play a vital role in driving innovation and solving important problems in our modern world.

## Conclusion

### Evaluating Inequalities of Combined Functions by Comparing the Functions Graphically

Evaluating Inequalities of Combined Functions by Comparing the Functions Graphically

Let $f(x) = 2x^{2}$ and $g(x) = 3x + 2$.

Graph the functions on the same set of axes. Identify the points of intersection.

#### Step 1: Graphing $f(x) = 2x^2$

To graph the function $f(x) = 2x^2$, we start by creating a table of values. This helps us plot the points accurately on the graph. Let's choose the values $x = -2, -1, 0, 1, 2$.

• For $x = -2$: $f(-2) = 2(-2)^2 = 2 \cdot 4 = 8$
• For $x = -1$: $f(-1) = 2(-1)^2 = 2 \cdot 1 = 2$
• For $x = 0$: $f(0) = 2(0)^2 = 0$
• For $x = 1$: $f(1) = 2(1)^2 = 2$
• For $x = 2$: $f(2) = 2(2)^2 = 8$

Plot these points $(-2, 8), (-1, 2), (0, 0), (1, 2), (2, 8)$ on the graph and connect them to form a parabola.

#### Step 2: Graphing $g(x) = 3x + 2$

Next, we graph the linear function $g(x) = 3x + 2$. Again, we use a table of values to find points to plot. Let's choose $x = -1, 0, 1, 2$.

• For $x = -1$: $g(-1) = 3(-1) + 2 = -3 + 2 = -1$
• For $x = 0$: $g(0) = 3(0) + 2 = 2$
• For $x = 1$: $g(1) = 3(1) + 2 = 3 + 2 = 5$
• For $x = 2$: $g(2) = 3(2) + 2 = 6 + 2 = 8$

Plot these points $(-1, -1), (0, 2), (1, 5), (2, 8)$ on the graph and connect them to form a straight line.

#### Step 3: Identifying Points of Intersection

To find the points of intersection between $f(x)$ and $g(x)$, we set the equations equal to each other: $2x^2 = 3x + 2$. Solving this equation will give us the x-coordinates of the intersection points.

Rearrange the equation: $2x^2 - 3x - 2 = 0$. We solve this quadratic equation using the factorization method:

• Factorize: $(2x + 1)(x - 2) = 0$
• Set each factor to zero: $2x + 1 = 0$ and $x - 2 = 0$
• Solve for $x$: $x = -\frac{1}{2}$ and $x = 2$

These x-values are the points where the functions intersect. To find the corresponding y-values, substitute $x = -\frac{1}{2}$ and $x = 2$ back into either $f(x)$ or $g(x)$.

• For $x = -\frac{1}{2}$: $f(-\frac{1}{2}) = 2(-\frac{1}{2})^2 = 2 \cdot \frac{1}{4} = \frac{1}{2}$
• For $x = 2$: $f(2) = 2(2)^2 = 8$

Thus, the points of intersection are $(- \frac{1}{2}, \frac{1}{2})$ and $(2, 8)$.

#### Step 4: Analyzing the Inequalities

With the points of intersection identified, we can now analyze the inequalities. We need to determine where f(x) > g(x) and f(x) < g(x).

By observing the graph:

• For x < -\frac{1}{2} and x > 2, f(x) > g(x).
• For -\frac{1}{2} < x < 2, f(x) < g(x).

This analysis helps us understand the regions where one function is greater than the other.

### FAQs

#### 1. What are the main approaches to solving inequalities of combined functions?

The three main approaches are: (1) Graphical comparison, which involves visualizing the functions on a coordinate plane; (2) Analyzing the difference function, which examines f(x) - g(x); and (3) Analyzing the quotient function, which looks at f(x) / g(x).

#### 2. When is the graphical approach most useful?

The graphical approach is most useful when dealing with simple functions or when you need a quick, intuitive understanding of the problem. It's particularly effective for functions that are easy to graph and when the solution can be visually identified.

#### 3. How does the difference function method work?

The difference function method involves creating a new function h(x) = f(x) - g(x). By analyzing where h(x) is positive, negative, or zero, we can determine the intervals where f(x) > g(x), f(x) < g(x), or f(x) = g(x), respectively.

#### 4. What precautions should be taken when using the quotient function approach?

When using the quotient function approach, it's crucial to be aware of potential division by zero. You should identify all x-values where the denominator (g(x)) equals zero, exclude these points from the analysis, and examine the behavior of the functions near these points separately.

#### 5. How do you choose the right approach for solving inequalities of combined functions?

The choice depends on the nature of the problem and the given functions. Use the graphical approach for simple functions and visual understanding, the difference function for problems involving maximizing or minimizing differences, and the quotient function for problems involving rates or ratios. Sometimes, a combination of approaches may be most effective for complex problems.

### Prerequisite Topics for Inequalities of Combined Functions

Understanding inequalities of combined functions requires a solid foundation in several key mathematical concepts. One of the most fundamental prerequisites is the domain and range of a function. This concept is crucial because when dealing with combined functions, you need to consider how the domains and ranges of individual functions interact and affect the overall inequality.

Another essential prerequisite is solving multi-step linear inequalities. This skill forms the basis for tackling more complex inequalities involving combined functions. By mastering linear inequalities, students can more easily grasp the techniques needed for solving inequalities with multiple functions.

Knowledge of quadratic functions in general form is also vital. Many combined function inequalities involve quadratic expressions, and understanding their behavior is key to solving these problems. This ties in closely with the ability to solve quadratic inequalities, which is often a stepping stone to working with more complex combined function inequalities.

The concept of combining transformations of functions is directly related to inequalities of combined functions. This skill helps students understand how different functions interact when combined, which is essential for analyzing and solving inequalities involving multiple functions.

Understanding the relations between x- and y-intercepts is crucial for graphing combined functions and their inequalities. These intercepts often provide key information about the solution set of an inequality.

The ability to solve rational inequalities is another important prerequisite. Combined functions often include rational expressions, and the techniques used for solving rational inequalities are frequently applied in more complex scenarios involving combined functions.

Lastly, familiarity with applications of polynomial functions provides real-world context for inequalities of combined functions. This knowledge helps students understand the practical significance of these mathematical concepts and how they can be applied to solve real-world problems.

By mastering these prerequisite topics, students will be well-equipped to tackle the challenges presented by inequalities of combined functions. Each concept builds upon the others, creating a comprehensive understanding that is essential for success in this advanced mathematical topic.

Difference function:

$f(x)$ > $g(x)$$f(x) - g (x)$ > $0$

Quotient function:

$f(x)$ > $g(x)$$\frac{f(x)}{g(x)}$ > $1$