Antiderivative Of Absolute Value Of X

The antiderivative of the absolute value of x, denoted as ∫|x| dx, presents a unique challenge due to the piecewise nature of the absolute value function. Understanding this antiderivative requires a careful consideration of how the absolute value function behaves for positive and negative values of x. This article provides a comprehensive exploration of the antiderivative of |x|, detailing the mathematical concepts, the step-by-step derivation, practical examples, and frequently asked questions to ensure a thorough understanding.

Understanding the Absolute Value Function

The absolute value function, |x|, is defined as:

|x| =

• x, if x ≥ 0
• -x, if x < 0

This means that |x| returns the non-negative value of x. For instance, |3| = 3 and |-3| = 3. This piecewise definition is crucial when finding the antiderivative because we need to consider both cases separately.

The Antiderivative Concept

An antiderivative of a function f(x) is a function F(x) such that F′(x) = f(x). In other words, the antiderivative is the reverse process of differentiation. When we find an antiderivative, we are essentially asking: "What function, when differentiated, gives us the function we started with?"

For example, the antiderivative of x is (x^2)/2 + C, where C is the constant of integration. This constant is added because the derivative of a constant is always zero, meaning there are infinitely many antiderivatives that differ only by a constant.

Deriving the Antiderivative of |x|

Given the piecewise nature of |x|, we need to find antiderivatives for both cases: x ≥ 0 and x < 0.

Case 1: x ≥ 0

When x ≥ 0, |x| = x. Therefore, we need to find the antiderivative of x:

∫ x dx = (x^2)/2 + C₁

Here, C₁ is the constant of integration for this case.

Case 2: x < 0

When x < 0, |x| = -x. Therefore, we need to find the antiderivative of -x:

∫ -x dx = -(x^2)/2 + C₂

Here, C₂ is the constant of integration for this case.

Combining the Cases

Now, we have two separate antiderivatives:

• (x^2)/2 + C₁, for x ≥ 0
• -(x^2)/2 + C₂, for x < 0

To express this as a single function, we need to ensure that the two pieces connect smoothly at x = 0. This means that the function must be continuous at x = 0.

Let's define the antiderivative F(x) as:

F(x) =

• (x^2)/2 + C₁, if x ≥ 0
• -(x^2)/2 + C₂, if x < 0

For F(x) to be continuous at x = 0, we need:

lim (x→0⁻) F(x) = lim (x→0⁺) F(x) = F(0)

This implies:

-(0^2)/2 + C₂ = (0^2)/2 + C₁

C₂ = C₁

Let C = C₁ = C₂. Now we have:

F(x) =

• (x^2)/2 + C, if x ≥ 0
• -(x2)/2 + C, if x < 0

A More Compact Form

To write this in a more compact and usable form, we can use the function (x|x|)/2:

• If x ≥ 0, then (x|x|)/2 = (x x)/2 = (x^2)/2
• If x < 0, then (x|x|)/2 = (x (-x))/2 = -(x^2)/2

Thus, the antiderivative of |x| can be written as:

∫ |x| dx = (x|x|)/2 + C

This single expression covers both cases and ensures continuity.

Verification Through Differentiation

To verify that our antiderivative is correct, we can differentiate (x|x|)/2 + C and check if we get |x|.

Case 1: x > 0

If x > 0, then |x| = x. So, F(x) = (x^2)/2 + C. The derivative is:

F′(x) = d/ dx [(x^2)/2 + C] = x

Since x > 0, F′(x) = x = |x|.

Case 2: x < 0

If x < 0, then |x| = -x. So, F(x) = -(x^2)/2 + C. The derivative is:

F′(x) = d/ dx [-(x^2)/2 + C] = -x

Since x < 0, F′(x) = -x = |x|.

Case 3: x = 0

At x = 0, we need to check the differentiability carefully. We can use the definition of the derivative:

F′(0) = lim (h→0) [(F(0 + h) - F(0)) / h]

We need to consider the left-hand limit and the right-hand limit separately.

• Right-hand limit (h > 0):

lim (h→0⁺) [((h|h|)/2 + C - (0|0|)/2 - C) / h] = lim (h→0⁺) [(h^2)/2h] = lim (h→0⁺) [h/2] = 0

• Left-hand limit (h < 0):

lim (h→0⁻) [((h|h|)/2 + C - (0|0|)/2 - C) / h] = lim (h→0⁻) [(-h^2)/2h] = lim (h→0⁻) [-h/2] = 0

Since both limits exist and are equal to 0, F′(0) = 0. Also, |0| = 0, so the derivative at x = 0 matches the absolute value function.

Therefore, the derivative of (x|x|)/2 + C is indeed |x| for all x, confirming that it is the correct antiderivative.

Definite Integrals

Now that we have the antiderivative, we can use it to evaluate definite integrals of the absolute value function. A definite integral is an integral with specified upper and lower limits, which gives a numerical value representing the area under the curve between those limits.

Example 1: ∫[-1, 2] |x| dx

To evaluate this definite integral, we use the antiderivative (x|x|)/2 + C. The constant C cancels out when evaluating definite integrals, so we can ignore it.

∫[-1, 2] |x| dx = [(x|x|)/2] evaluated from -1 to 2

= [(2|2|)/2] - [(-1|-1|)/2]

= [(22)/2] - [(-11)/2]

= [4/2] - [-1/2]

= 2 + 1/2

= 5/2

So, ∫[-1, 2] |x| dx = 5/2.

Example 2: ∫[-3, -1] |x| dx

∫[-3, -1] |x| dx = [(x|x|)/2] evaluated from -3 to -1

= [(-1|-1|)/2] - [(-3|-3|)/2]

= [(-11)/2] - [(-33)/2]

= [-1/2] - [-9/2]

= -1/2 + 9/2

= 8/2

= 4

So, ∫[-3, -1] |x| dx = 4.

Example 3: ∫[0, 5] |x| dx

∫[0, 5] |x| dx = [(x|x|)/2] evaluated from 0 to 5

= [(5|5|)/2] - [(0|0|)/2]

= [(5*5)/2] - [0]

= 25/2

So, ∫[0, 5] |x| dx = 25/2.

Applications of the Antiderivative of |x|

The antiderivative of the absolute value of x has applications in various fields, including:

• Physics: In mechanics, it can be used to calculate the distance traveled by an object when the velocity is given as an absolute value function.
• Engineering: In signal processing, it can be used to analyze signals that are represented by absolute value functions.
• Economics: In economic models, it can be used to represent and analyze functions involving absolute deviations or errors.
• Mathematics: It serves as an important example in calculus for understanding antiderivatives of piecewise functions and dealing with continuity and differentiability.

Graphical Interpretation

The antiderivative F(x) = (x|x|)/2 + C represents a family of curves, each differing by the constant C. The graph of F(x) is a piecewise quadratic function. For x ≥ 0, it's a parabola opening upwards, and for x < 0, it's a parabola opening downwards. The two parabolas meet at x = 0, ensuring a smooth transition.

If you plot F(x) for different values of C, you'll see a vertical shift in the graph, but the overall shape remains the same. This reflects the fact that the derivative of F(x) is always |x|, regardless of the value of C.

Common Mistakes

When finding the antiderivative of |x|, several common mistakes can occur:

1. Ignoring the Piecewise Nature: The most common mistake is to treat |x| as a single function without considering its piecewise definition. This leads to incorrect antiderivatives.
2. Incorrectly Applying the Power Rule: Applying the power rule directly without considering the absolute value can lead to errors, especially for x < 0.
3. Forgetting the Constant of Integration: Forgetting to add the constant of integration C results in an incomplete antiderivative. Remember that the antiderivative is a family of functions, not just a single function.
4. Not Ensuring Continuity: Failing to ensure that the piecewise antiderivative is continuous at x = 0 can lead to incorrect results. The constants of integration must be chosen such that the antiderivative is continuous.

Advanced Concepts

Generalized Absolute Value Functions

The antiderivative of |x| can be extended to more complex absolute value functions, such as |f(x)|, where f(x) is a function of x. To find the antiderivative of |f(x)|, one needs to:

1. Identify the intervals where f(x) ≥ 0 and f(x) < 0.
2. Find the antiderivative of f(x) in the intervals where f(x) ≥ 0.
3. Find the antiderivative of -f(x) in the intervals where f(x) < 0.
4. Ensure that the piecewise antiderivative is continuous at the points where f(x) = 0.

Integration Techniques

In some cases, integration techniques such as substitution or integration by parts may be necessary to find the antiderivative of more complicated absolute value functions. These techniques can help simplify the integrand and make it easier to find the antiderivative.

Conclusion

The antiderivative of the absolute value of x is given by F(x) = (x|x|)/2 + C. This result is derived by considering the piecewise nature of the absolute value function and ensuring that the antiderivative is continuous. The antiderivative has various applications in physics, engineering, economics, and mathematics. By understanding the concepts and techniques presented in this article, one can confidently find and apply the antiderivative of |x| in a wide range of problems.

Frequently Asked Questions (FAQ)

1. What is the antiderivative of |x|?

The antiderivative of |x| is (x|x|)/2 + C, where C is the constant of integration.

2. Why do we need to consider cases for x ≥ 0 and x < 0?

Because the absolute value function |x| is defined piecewise as x for x ≥ 0 and -x for x < 0. To find the antiderivative, we need to treat each case separately.

3. How do we ensure continuity of the antiderivative at x = 0?

By choosing the constants of integration such that the values of the antiderivative from both cases are equal at x = 0. This ensures a smooth transition.

4. Can we use the power rule directly to find the antiderivative of |x|?

No, the power rule cannot be directly applied because |x| is not a simple power function. The piecewise nature of |x| requires a different approach.

5. What is a definite integral, and how do we evaluate it for |x|?

A definite integral is an integral with specified upper and lower limits. To evaluate it for |x|, we use the antiderivative (x|x|)/2 + C and apply the fundamental theorem of calculus.

6. What are some applications of the antiderivative of |x|?

It has applications in physics (mechanics), engineering (signal processing), economics (economic models), and mathematics (calculus).

7. What are some common mistakes when finding the antiderivative of |x|?

Common mistakes include ignoring the piecewise nature, incorrectly applying the power rule, forgetting the constant of integration, and not ensuring continuity.

8. How does the graph of the antiderivative look?

The graph of the antiderivative F(x) = (x|x|)/2 + C is a piecewise quadratic function, consisting of two parabolas that meet smoothly at x = 0.

9. Can we find the antiderivative of |f(x)| for any function f(x)?

Yes, but it requires identifying the intervals where f(x) ≥ 0 and f(x) < 0, and then finding the antiderivative of f(x) and -f(x) in those intervals, respectively.

10. What if the absolute value function is part of a more complex integral?

In such cases, integration techniques like substitution or integration by parts may be necessary to simplify the integrand and find the antiderivative.