Integration By Parts For Definite Integral

The integration by parts technique, a cornerstone of integral calculus, allows us to tackle integrals of products of functions by cleverly reversing the product rule of differentiation. When applying integration by parts to definite integrals, we introduce an added layer of evaluation at the limits of integration, making it a powerful tool for solving a wide variety of problems. This article delves into the intricacies of integration by parts for definite integrals, providing a comprehensive guide with illustrative examples and practical insights.

Understanding Integration by Parts

At its core, integration by parts stems from the product rule of differentiation. Recall that the product rule states:

d/dx [u(x)v(x)] = u'(x)v(x) + u(x)v'(x)

Integrating both sides with respect to x gives us:

∫ d/dx [u(x)v(x)] dx = ∫ u'(x)v(x) dx + ∫ u(x)v'(x) dx

u(x)v(x) = ∫ u'(x)v(x) dx + ∫ u(x)v'(x) dx

Rearranging this equation yields the fundamental formula for integration by parts:

∫ u(x)v'(x) dx = u(x)v(x) - ∫ v(x)u'(x) dx

In more concise notation:

∫ u dv = uv - ∫ v du

Integration by Parts for Definite Integrals: The Formula

For definite integrals, we simply evaluate the uv term at the upper and lower limits of integration. The formula becomes:

∫ab u dv = [uv]ab - ∫ab v du

Where:

• a is the lower limit of integration.
• b is the upper limit of integration.
• [uv]ab represents uv(b) - uv(a), the evaluation of the product uv at the upper limit b minus its evaluation at the lower limit a.

Steps for Applying Integration by Parts to Definite Integrals

Follow these steps to effectively use integration by parts for definite integrals:

1. Choose u and dv: The key is to strategically select u and dv from the integrand. A helpful mnemonic is LIATE, which prioritizes function types for choosing u:

   • Logarithmic functions (e.g., ln(x))
   • Inverse trigonometric functions (e.g., arctan(x))
   • Algebraic functions (e.g., x, x^2)
   • Trigonometric functions (e.g., sin(x), cos(x))
   • Exponential functions (e.g., e^x)

   Choose u based on this order – the function that appears higher on the list is generally a good choice for u. The remaining part of the integrand becomes dv.

2. Calculate du and v: Once you've chosen u and dv, find du by differentiating u with respect to x, and find v by integrating dv with respect to x.

3. Apply the Formula: Substitute u, v, du, and dv into the definite integral integration by parts formula:

   ∫ab u dv = [uv]ab - ∫ab v du

4. Evaluate the terms:

   • Evaluate [uv]ab, which is uv(b) - uv(a).
   • Evaluate the new integral ∫ab v du. This may require further integration techniques, including another application of integration by parts.

5. Simplify: Combine the evaluated terms and simplify the result to obtain the final answer.

Illustrative Examples

Let's work through several examples to solidify understanding.

Example 1: ∫0π/2 x cos(x) dx

1. Choose u and dv: Using LIATE, we choose u = x (algebraic) and dv = cos(x) dx (trigonometric).
2. Calculate du and v:
   • du = dx
   • v = ∫ cos(x) dx = sin(x)
3. Apply the Formula: ∫0π/2 x cos(x) dx = [x sin(x)]0π/2 - ∫0π/2 sin(x) dx
4. Evaluate the terms:
   • [x sin(x)]0π/2 = (π/2) sin(π/2) - (0) sin(0) = (π/2)(1) - 0 = π/2
   • ∫0π/2 sin(x) dx = [-cos(x)]0π/2 = -cos(π/2) - (-cos(0)) = -0 + 1 = 1
5. Simplify: ∫0π/2 x cos(x) dx = π/2 - 1

Example 2: ∫1e ln(x) dx

1. Choose u and dv: Using LIATE, we choose u = ln(x) (logarithmic) and dv = dx (algebraic – a constant function).
2. Calculate du and v:
   • du = (1/x) dx
   • v = ∫ dx = x
3. Apply the Formula: ∫1e ln(x) dx = [x ln(x)]1e - ∫1e x (1/x) dx
4. Evaluate the terms:
   • [x ln(x)]1e = (e) ln(e) - (1) ln(1) = e(1) - 1(0) = e
   • ∫1e x (1/x) dx = ∫1e 1 dx = [x]1e = e - 1
5. Simplify: ∫1e ln(x) dx = e - (e - 1) = 1

Example 3: ∫01 arctan(x) dx

1. Choose u and dv: Using LIATE, we choose u = arctan(x) (inverse trigonometric) and dv = dx (algebraic).
2. Calculate du and v:
   • du = (1/(1+x^2)) dx
   • v = ∫ dx = x
3. Apply the Formula: ∫01 arctan(x) dx = [x arctan(x)]01 - ∫01 x (1/(1+x^2)) dx
4. Evaluate the terms:
   • [x arctan(x)]01 = (1) arctan(1) - (0) arctan(0) = (1) (π/4) - 0 = π/4
   • To evaluate ∫01 x (1/(1+x^2)) dx, we use substitution. Let w = 1 + x^2, then dw = 2x dx, so (1/2) dw = x dx. The new limits of integration are: when x = 0, w = 1; when x = 1, w = 2. So, ∫01 x (1/(1+x^2)) dx = (1/2) ∫12 (1/w) dw = (1/2) [ln(w)]12 = (1/2) (ln(2) - ln(1)) = (1/2) ln(2)
5. Simplify: ∫01 arctan(x) dx = π/4 - (1/2) ln(2)

Example 4: ∫0π e^x cos(x) dx

This example requires applying integration by parts twice.

1. First Application:

   • Choose u = e^x and dv = cos(x) dx.
   • du = e^x dx and v = sin(x). ∫0π e^x cos(x) dx = [e^x sin(x)]0π - ∫0π e^x sin(x) dx [e^x sin(x)]0π = e^π sin(π) - e^0 sin(0) = 0 - 0 = 0 So, ∫0π e^x cos(x) dx = - ∫0π e^x sin(x) dx

2. Second Application:

   • Now, we need to integrate ∫0π e^x sin(x) dx. Choose u = e^x and dv = sin(x) dx.
   • du = e^x dx and v = -cos(x). ∫0π e^x sin(x) dx = [-e^x cos(x)]0π - ∫0π -e^x cos(x) dx [-e^x cos(x)]0π = -e^π cos(π) - (-e^0 cos(0)) = -e^π (-1) + 1 = e^π + 1 So, ∫0π e^x sin(x) dx = e^π + 1 + ∫0π e^x cos(x) dx

3. Solve for the Original Integral: Let I = ∫0π e^x cos(x) dx. From the first application: I = - ∫0π e^x sin(x) dx From the second application: ∫0π e^x sin(x) dx = e^π + 1 + I Substitute the second result into the first: I = - (e^π + 1 + I) I = -e^π - 1 - I 2I = -e^π - 1 I = (-e^π - 1) / 2

   Therefore, ∫0π e^x cos(x) dx = (-e^π - 1) / 2

Advanced Considerations and Techniques

• Tabular Integration: When dealing with repeated integration by parts, tabular integration (also known as the "Tic-Tac-Toe" method) can significantly simplify the process. This is especially useful when one function can be repeatedly differentiated to zero (e.g., polynomials).
• Cyclic Integrals: As seen in Example 4, some integrals, like ∫ e^x cos(x) dx, are cyclic. Applying integration by parts twice returns the original integral, allowing you to solve for the integral algebraically.
• Choosing u Strategically: The choice of u is critical. If you choose poorly, the new integral ∫ v du might be more complicated than the original. Practice and familiarity with LIATE will help you make optimal choices.
• Dealing with Discontinuities: If the integrand or its derivatives have discontinuities within the interval of integration, you may need to split the integral into multiple integrals around the points of discontinuity.

Common Mistakes to Avoid

• Incorrectly Applying the Formula: Double-check the formula for integration by parts to ensure you have the correct signs and terms.
• Forgetting to Evaluate uv at the Limits: A common mistake is to forget to evaluate the uv term at the upper and lower limits of integration in definite integrals.
• Incorrect Integration or Differentiation: Errors in finding du or v will propagate through the entire solution.
• Choosing the Wrong u: A poor choice of u can lead to a more complex integral. Use LIATE as a guide.
• Not Simplifying: Always simplify the final expression to its simplest form.

Applications of Integration by Parts

Integration by parts is not just a theoretical exercise; it has numerous applications in various fields, including:

• Physics: Calculating moments of inertia, analyzing damped oscillations, and solving problems in electromagnetism.
• Engineering: Determining centroids and moments of areas, analyzing signal processing, and modeling control systems.
• Probability and Statistics: Finding expected values and variances of continuous random variables.
• Economics: Modeling present and future values of investments and analyzing economic growth models.

Practice Problems

To master integration by parts for definite integrals, practice is essential. Here are some problems to try:

1. ∫01 x e^x dx
2. ∫12 x^3 ln(x) dx
3. ∫0π/4 x sec^2(x) dx
4. ∫01 arcsin(x) dx
5. ∫1e (ln(x))^2 dx

Conclusion

Integration by parts is a powerful and versatile technique for evaluating integrals, especially those involving products of functions. For definite integrals, it introduces the additional step of evaluating the uv term at the limits of integration. By carefully choosing u and dv, applying the formula correctly, and avoiding common mistakes, you can successfully tackle a wide range of integration problems. Through consistent practice and a solid understanding of the underlying principles, integration by parts will become an indispensable tool in your calculus toolkit. Mastering this technique opens doors to solving complex problems in various scientific and engineering disciplines. Remember the mnemonic LIATE, pay close attention to detail, and never underestimate the power of simplification. With dedication and perseverance, you'll be well-equipped to conquer even the most challenging integration by parts problems.