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IB DP Maths AA SL Study Notes

5.5.2 Integration by Partial Fractions

What are Partial Fractions?

In the realm of maths, when we discuss partial fractions, we're referring to the method of expressing a given rational function as a sum of simpler fractions. This decomposition is a strategic move, aiming to transform a complex fraction into fractions with more straightforward denominators. Once this is achieved, each of these simpler fractions can be integrated individually, making the overall process more efficient.

The Decomposition Process

The process of breaking down a rational function into its constituent partial fractions involves several systematic steps:

1. Factorising the Denominator: The first step requires you to factorise the denominator of the rational function. This means breaking it down into its irreducible factors.

2. Establishing the General Form: Depending on the factors you've derived from the denominator, you'll need to set up a general form for the partial fractions. Here's a quick guide:

  • For linear factors (like x or x+1), use terms such as A/x or B/(x+1).
  • For repeated linear factors (like x2 or x3), the terms would look something like A/x + B/x2.
  • For quadratic factors that can't be factorised further, use terms like (Ax+B)/(x2+1).

3. Equating Coefficients: This step involves a bit of algebraic manipulation. Multiply through by the common denominator to eliminate the fractions. Once that's done, equate the coefficients of similar terms on both sides. This will give you a system of equations that you can solve to determine the constants A, B, etc.

4. Combining the Fractions: With the constants in hand, you can now rewrite the original fraction as a sum of its partial fractions.

Applications in Integration

The primary reason for using partial fractions in calculus is to simplify integration. By breaking down a complex rational function, each part can be integrated separately, often using basic integration techniques. This is especially handy when the integrand is a complex rational function.

Detailed Example:

Consider the function 2x/(x2 + x - 6).

To integrate this, we first decompose the fraction. Factorising the denominator, we get (x-2)(x+3). Now, we can express the fraction as: 2x/(x2 + x - 6) = A/(x-2) + B/(x+3)

Multiplying through by the common denominator, we get: 2x = A(x+3) + B(x-2)

By equating coefficients and solving for A and B, we find their values. Once we have A and B, we can integrate each term separately to get the result.

Another example is the function (x2 + 4)/(x3 + 3x2).

Breaking down this fraction using partial fractions, we can express it in terms of simpler fractions and then integrate each term. The integration process for each term will involve basic techniques, and the final result will be a combination of the integrals of these simpler fractions.

Practice Questions:

1. Integrate the function 3x/(x2 - 4x + 4).Solution: Start by decomposing the fraction into partial fractions. Once that's done, integrate each term separately. The combined result will give you the integral of the original function.

2. Evaluate the integral of (2x2 + 5)/(x3 - x2 + x).Solution: Decompose the fraction using partial fractions. Then, integrate each term. The final result will be a combination of the integrals of these simpler fractions. 

FAQ

When faced with irreducible quadratic factors in the denominator, the corresponding term in the partial fraction decomposition will have a linear numerator. For example, if the denominator has a factor like x2 + 1, the corresponding term in the decomposition will be of the form (Ax+B)/(x2+1). Here, A and B are constants that can be determined by equating coefficients, and the integration will typically involve trigonometric or hyperbolic substitution.

The method of partial fractions is specifically tailored for rational functions, which are ratios of polynomials. While the technique is most commonly applied to polynomial ratios, it can be extended to some other functions with appropriate modifications. However, for functions outside the realm of rational polynomials, other integration techniques might be more suitable or straightforward. It's essential to choose the method that best fits the function at hand.

Yes, there are certain conditions that must be met to use the method of partial fractions. The primary condition is that the degree of the numerator polynomial must be less than the degree of the denominator polynomial. If this isn't the case, polynomial long division must first be applied to reduce the numerator's degree. Only once this condition is satisfied can we proceed with the decomposition into partial fractions.

The method of partial fractions is specifically designed for rational functions because these functions are ratios of two polynomials. The primary goal of this method is to decompose a complex rational function into simpler fractions that are easier to integrate. By breaking down the function into its constituent parts, each with simpler denominators, the integration process becomes more straightforward. This decomposition allows us to tackle each simpler fraction individually, often using basic integration techniques, making the overall process more efficient.

Absolutely! When the denominator has repeated roots, the partial fraction decomposition will have terms corresponding to each power of the repeated factor. For instance, if the denominator has a factor of (x-1)3, the decomposition will include terms like A/(x-1), B/(x-1)2, and C/(x-1)3. Each of these terms will be integrated separately, and the constants A, B, and C will be determined by equating coefficients.

Practice Questions

Evaluate the integral of the function 1/(x^2 + 2x + 1) with respect to x.

To integrate the function 1/(x2 + 2x + 1), we recognise the denominator as a perfect square trinomial, which can be written as (x+1)2. The integral becomes the integral of 1/(x+1)2. Integrating this expression, we get -1/(x+1) + C, where C is the constant of integration.

Determine the integral of the function 2x/(x^2 - 3x + 2) with respect to x.

To tackle the function 2x/(x2 - 3x + 2), we first factorise the denominator to get (x-2)(x-1). Using partial fraction decomposition, we can express the function in terms of simpler fractions and then integrate each term. The integral results in 4 * ln|2 - x| - 2 * ln|1 - x| + C, where C is the constant of integration.

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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