Question

True or False The cube function is odd and is increasing on the interval \((-\infty, \infty)\)

Short answer

True. The cube function is odd and is non-decreasing on \((-fty, fty)\).

Step-by-step solution

Understanding the Cube Function

The cube function is defined as \( f(x) = x^3 \). We need to determine whether this function is odd and whether it is increasing on the interval \((-fty, fty)\).

Test for Odd Function

A function \(f(x)\) is odd if \(f(-x) = -f(x)\) for all \(x\). Let's compute \( f(-x) \) for \( f(x) = x^3 \): \[ f(-x) = (-x)^3 = -x^3 = -f(x) \]. Since \( f(-x) = -f(x) \), the cube function is indeed odd.

Test for Monotonic Increase

A function is increasing if its derivative is positive for all \(x\). The derivative of \(f(x) = x^3\) is \(f'(x) = 3x^2\). Since \(3x^2 \geq 0\) for all \(x\) and is only zero at \(x = 0\), the derivative is either zero or positive everywhere. Thus, the cube function is non-decreasing on \((-fty, fty)\).

Key concepts

Odd Function

An odd function has a specific symmetry property: for every input value, the output of the function should satisfy the condition: \(f(-x) = -f(x)\). This means that if you take any point on the graph of the function and reflect it across the origin, you should get another point on the graph.

To see this in action with the cube function, let's consider its definition: \(f(x) = x^3\). If we check \f(-x)\, we obtain \f(-x) = (-x)^3 = -x^3\. Notice that this result is exactly \f(-x) = -f(x)\, confirming that the cube function is odd. This symmetry tells us that the function behaves the same way in both the positive and negative directions, which is a unique and important characteristic of odd functions.

Monotonic Increase

A function is monotonically increasing if its values always increase as the input value increases. For a function to be monotonically increasing across its entire domain, its derivative must be greater than or equal to zero for all input values.

For the cube function \(f(x) = x^3\), we first find its derivative, \f'(x)\. The derivative of \x^3\ is \3x^2\. Now, consider this derivative: \(3x^2 ≥ 0\) for all \x\. The term \3x^2\ is always non-negative because the square of any real number is non-negative. The only time it equals zero is when \x = 0\, but this doesn't make the function decrease anywhere.

This property confirms that the cube function is *non-decreasing* everywhere on \(-∞, ∞)\). Even though it does not strictly increase at \x = 0\, the function doesn't decrease at any point, ensuring it is indeed increasing (in the monotonic sense) over the entire interval.

Derivative Analysis

The derivative of a function provides us crucial information about its behavior, particularly in understanding whether the function is increasing or decreasing at a given point or interval. Let's break down the derivative analysis for the cube function.

Firstly, we define the cube function: \(f(x) = x^3\). Calculating its derivative gives us: \(f'(x) = 3x^2\). This derivative tells us how the slope of the tangent to the function changes with \x\.

Analyzing \f'(x) = 3x^2\ reveals two important points:

• \3x^2\geq 0\ for all \x\, indicating that the function is non-decreasing.
• The derivative is zero only at \x = 0\, meaning there is no change in the slope at this exact point. Everywhere else, the slope is positive.

Having a non-negative derivative everywhere shows that the function is increasing overall, although it does not decrease at any point. This derivative analysis is critical for identifying the nature of the function's rate of change and ensures a comprehensive understanding of its behavior across its entire domain.