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IB DP Maths AA SL Study Notes

5.5.1 Integration by Parts

Historical Insight

The roots of the integration by parts technique can be traced back to the method of "exhaustion" employed by ancient Greek mathematicians. However, its modern form was shaped in the 17th century by the likes of Isaac Newton and Gottfried Wilhelm Leibniz, who are celebrated for their contributions to calculus.

The Core Formula and Its Origin

The formula for integration by parts is articulated as:

Integrate u dv = uv - Integrate v du

Here:

  • u represents the function we opt to differentiate.
  • dv signifies the function we decide to integrate.

This formula emerges from the product rule for differentiation:

d(uv) = u dv + v du

On integrating both sides, we deduce:

uv = Integrate u dv + Integrate v du

Rearranging the terms gives us the integration by parts formula.

Picking u and dv

The efficacy of this method often hinges on the apt choice of u and dv. A popular mnemonic to assist in this selection is LIATE:

  • Logarithmic functions
  • Inverse trigonometric functions
  • Algebraic functions
  • Trigonometric functions
  • Exponential functions

The sequence indicates the preference for selecting u. For instance, if you're juggling between a logarithmic and an algebraic function, the logarithmic one should be your pick for u.

Diving Deeper into Applications

1. Tackling Products of Functions

The primary application arena for integration by parts is integrals that involve products of functions.

Example:

To evaluate Integrate x sin(x) dx:

Using the LIATE guideline, we select: u = x (Algebraic function) and dv = sin(x) dx (Trigonometric function).

On differentiation and integration, we deduce: du = dx and v = -cos(x).

Plugging into the formula: Integrate x sin(x) dx = -x cos(x) - Integrate (-cos(x)) dx This simplifies to: -x cos(x) + sin(x) + C, where C is the integration constant.

2. The Art of Repeated Application

Certain integrals demand that the integration by parts method be invoked more than once.

Example:

To evaluate Integrate ex sin(x) dx:

This particular integral calls for the method to be applied twice. By setting up two integrations by parts and amalgamating the outcomes, we can decipher the original integral.

3. Navigating Through Complex Integrals

The realm of integration by parts isn't confined to elementary functions. It can be wielded for more intricate integrals, often simplifying them substantially.

Practice Questions and Walkthroughs:

1. Evaluate Integrate x2 ln(x) dx.

Solution: By employing integration by parts, the integral evaluates to -1/9 x3 + x3 ln(x)/3 + C.

2. Compute Integrate x cos(2x) dx.

Solution: The outcome of this integral is cos(2x)/4 + x sin(2x)/2 + C.

3. Ascertain the value of Integrate ex cos(x) dx.

Solution: The integral boils down to (ex (cos(x) + sin(x)))/2 + C.

Pro Tips for Mastery

  • Regular practice is the golden key. Consistently tackling a diverse set of problems will cement your grasp.
  • If the resultant integral post-application seems more convoluted than the original, rethink your choices for u and dv.
  • The LIATE rule is your compass. It's an invaluable guide steering your decisions.

FAQ

In some cases, after applying integration by parts once, the resulting integral is still not in a form that can be easily integrated. However, it might be in a form that allows for another application of integration by parts. By applying the method repeatedly, we can sometimes reduce the integral to a more manageable form or even to an integral that cancels out terms from the original equation. This iterative process can simplify the problem, leading to a solvable integral or a recursive relationship that can be used to determine the original integral's value.

Yes, there are integrals that cannot be directly solved using integration by parts, or where the method might not be the most efficient approach. Integration by parts is particularly useful for integrals involving products of functions. However, for some functions, the method might lead to an integral that's as complex or even more complex than the original. In such cases, other techniques like trigonometric substitution, partial fraction decomposition, or even numerical methods might be more appropriate. It's crucial to have a repertoire of integration techniques and to choose the best one based on the specific integral at hand.

Integration by parts is essentially the reverse application of the product rule for differentiation. The product rule states that the derivative of a product of two functions is the first function times the derivative of the second plus the second function times the derivative of the first. When you rearrange this rule and integrate both sides, you derive the formula for integration by parts. In essence, while the product rule gives a method to differentiate products of functions, integration by parts provides a technique to integrate them.

The LIATE rule is a mnemonic used to help students decide which function to differentiate (u) and which to integrate (dv) when applying the integration by parts method. The order in LIATE (Logarithmic, Inverse trigonometric, Algebraic, Trigonometric, Exponential) suggests a general preference, but it's essential to understand that it's more of a guideline than a strict rule. There are certainly exceptions, and sometimes, based on the specific integral, it might be more beneficial to deviate from the LIATE order. The rule is there to provide a starting point, but with experience, students will develop an intuition for choosing u and dv effectively.

Absolutely! Integration by parts can be applied to both indefinite and definite integrals. When applied to definite integrals, the process remains largely the same, but with an added step at the end. After finding the antiderivative using integration by parts, you'll evaluate it at the upper and lower limits of integration and subtract the two results. This gives the total accumulated value over the interval, which is the value of the definite integral. It's a powerful technique that extends the utility of integration by parts beyond just finding antiderivatives.

Practice Questions

Evaluate the integral of x^3 multiplied by e^x with respect to x.

To tackle this integral, we can use the integration by parts technique. By selecting u as x3 and dv as ex dx, we differentiate and integrate respectively to obtain du as 3x2 dx and v as ex. Using the integration by parts formula, we deduce that the integral is equivalent to: ex times (x3 - 3x2 + 6x - 6).

Determine the integral of ln(x) multiplied by cos(x) with respect to x.

For this particular integral, the integration by parts method comes in handy. By opting for u as ln(x) and dv as cos(x) dx, we differentiate and integrate to get du as 1/x dx and v as sin(x). Applying the formula, the integral translates to: cos of ln of x3 divided by 3. This result represents the integral of the product of the natural logarithm of x and the cosine of x.

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
LinkedIn
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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