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Methods of Integration> Integration Using Partial Fraction

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Estimated time: 9 minutes
CBSE: Class 12

Definition: Integration Using Partial Fraction

Integration by partial fractions is a method used to integrate rational functions, that is, functions of the form

\[\frac{p(x)}{q(x)}\], where both numerator and denominator are polynomials.
CBSE: Class 12
Maharashtra State Board: Class 12

Types of Partial Fractions

Type Rational Form Partial Form
Type I (Non-repeated linear factors) \[\frac{\mathrm{p}x+\mathrm{q}}{(x-\mathrm{a})(x-\mathrm{b})}\] \[\frac{\mathrm{A}}{x-\mathrm{a}}+\frac{\mathrm{B}}{x-\mathrm{b}}\]
\[\frac{\mathrm{p}x^{2}+\mathrm{q}x+\mathrm{r}}{(x-\mathrm{a})(x-\mathrm{b})(x-\mathrm{c})}\] \[\frac{\mathrm{A}}{x-\mathrm{a}}+\frac{\mathrm{B}}{x-\mathrm{b}}+\frac{\mathrm{C}}{x-\mathrm{c}}\]
Type II (Repeated linear factors) \[\frac{\mathrm{p}x+\mathrm{q}}{\left(x-\mathrm{a}\right)^2}\] \[\frac{\mathrm{A}}{(x-\mathrm{a})}+\frac{\mathrm{B}}{(x-\mathrm{a})^{2}}\]
\[\frac{\mathrm{p}x^{2}+\mathrm{q}x+\mathrm{r}}{\left(x-\mathrm{a}\right)^{2}\left(x-\mathrm{b}\right)}\] \[\frac{\mathrm{A}}{(x-\mathrm{a})}+\frac{\mathrm{B}}{(x-\mathrm{a})^{2}}+\frac{\mathrm{C}}{(x-\mathrm{b})}\]
Type III (Linear × Quadratic) \[\frac{\mathrm{p}x^{2}+\mathrm{q}x+\mathrm{r}}{(x-\mathrm{a})(x^{2}+\mathrm{b}x+\mathrm{c})}\] \[\frac{\mathrm{A}}{(x-\mathrm{a})}+\frac{\mathrm{B}x+\mathrm{C}}{(x^{2}+\mathrm{b}x+\mathrm{c})}\]
CBSE: Class 12

Example 1

Find \[\int \frac{x^2 + 1}{x^2 - 5x + 6} dx\]

Solution:

Step 1: Divide

Here the integrand \[\frac{x^2 + 1}{x^2 - 5x + 6}\] is not proper rational function

Step 2: Factor denominator

\[x^2 + 1\] - \[x^2 - 5x + 6\] = 5x - 5

\[\frac{x^2 + 1}{x^2 - 5x + 6} = 1 + \frac{5x - 5}{x^2 - 5x + 6} = 1 + \frac{5x - 5}{(x - 2)(x - 3)}\]

Let \[\frac{5x - 5}{(x - 2)(x - 3)} = \frac{\text{A}}{x - 2} + \frac{\text{B}}{x - 3}\]

Step 3: Find A and B

 \[5x - 5 = \text{A}(x - 3) + \text{B}(x - 2)\]

Equating the coefficients of \[x\] and constant terms on both sides, we get \[\text{A} + \text{B} = 5\] and \[3\text{A} + 2\text{B} = 5\]. Solving these equations, we get 

B = 5 − A

Substitute into the second equation:

3A + 2(5−A) = 5

3A + 10 − 2A = 5

A = −5 & B = 10

Thus, \[\frac{x^2 + 1}{x^2 - 5x + 6} = 1 - \frac{5}{x - 2} + \frac{10}{x - 3}\]

Step 4: Rewrite integral

Therefore, \[\int \frac{x^2 + 1}{x^2 - 5x + 6} dx = \int dx - 5 \int \frac{1}{x - 2} dx + 10 \int \frac{dx}{x - 3}\]

Step 5: Integrate

\[= x - 5 \log |x - 2| + 10 \log |x - 3| + \text{C}\].

CBSE: Class 12
Maharashtra State Board: Class 12

Key Points : Partial Fractions

  • First check whether the rational function is proper or improper.

  • Use long division before decomposition if the fraction is improper.

  • Factorise the denominator completely before choosing partial fractions.

  • For each distinct linear factor, use a constant numerator such as A, B, or C.

  • For a repeated linear factor, include every power separately.

  • For an irreducible quadratic factor, use a linear numerator of the form Bx + C.

  • After decomposition, integrate each term separately using standard formulas.

Video Tutorials

We have provided more than 1 series of video tutorials for some topics to help you get a better understanding of the topic.

Series 1


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Shaalaa.com | Indefinite Integral Part 11 (Integrals using Partial Fraction)

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