Problem 39 - Algebra and Trigonometry

Question

Find the domain of the function. $$ f(x)=2 x, \quad-1 \leq x \leq 5 $$

Step-by-Step Solution

Answer

The domain is \([-1, 5]\).

Step 1: Understand the Function

The function given is \( f(x) = 2x \), a linear function. Linear functions are continuous and defined for all real numbers unless specified otherwise.

Step 2: Identify Domain Restrictions

The exercise states the condition \(-1 \leq x \leq 5\), which limits \(x\) to values between -1 and 5, inclusive.

Step 3: Write the Domain in Interval Notation

Based on the restriction \(-1 \leq x \leq 5\), we express the domain in interval notation as \([-1, 5]\).

Key Concepts

Understanding Linear Functions

A linear function is simply a function that creates a straight line when graphed on the coordinate plane. They're often expressed in the form \( f(x) = ax + b \), where \( a \) and \( b \) are constants. The beauty of linear functions is their straightforward behavior. They have a constant rate of change, which means for each unit increase in \( x \), there is a constant change in \( f(x) \).

Linear functions are among the simplest functions in mathematics because they have two main characteristics:

• Constant Slope: The "slope" \( a \) in \( ax + b \) indicates how steep the line is. It’s constant, meaning it does not change no matter where you are on the line.
• Unbounded: Typically, linear functions are defined for all real numbers, unless restricted by specific conditions.

A simple example of a linear function is \( f(x) = 2x \), which is a linear equation that doubles every \( x \) value. However, in some cases, like in our exercise, linear functions can be restricted to a specific interval, such as \(-1 \leq x \leq 5\).

Continuous Functions Explained

Continuous functions are functions that are unbroken for every value within their domain. Imagine you were graphing the function; you could draw the curve without ever lifting your pencil from the paper. This property is crucial in understanding many function types.

For linear functions like \( f(x) = 2x \), the function is naturally continuous over its domain because the graph is a line without any interruptions. Even if it's defined on a restricted interval, the line remains smooth and unbroken between its endpoints.

The significance of continuity includes:

• Predictability: Continuous functions allow for accurate predictions at any point within their domain.
• No gaps or jumps: Ensure smooth transitions between values, which is helpful for optimization problems.

A function being continuous also means every sequence of \( x \) values approaching a point give sequence of \( f(x) \) values approaching \( f \) at that point.

Utilizing Interval Notation

Interval notation is a method for expressing the set of values that make up the domain of a function. It's a concise way to show which "pieces" of the number line the function is concerned with. In interval notation, parentheses \((\) and brackets \([\) play crucial roles:

• Parentheses \((\): Indicate that a boundary number is not included in the interval.
• Brackets \([\): Indicate that a boundary number is included in the interval.

To write the domain of a function like \( f(x) = 2x \), where \(-1 \leq x \leq 5\), we use interval notation \([-1, 5]\). Here, the square brackets indicate that both -1 and 5 are included as part of the domain.

Using interval notation provides a sleek and clear way to communicate such restrictions without cumbersome words. It’s particularly useful in higher-level mathematics, where it’s important to clearly define where a function is applicable.