Indefinite Integration
Integration of Rational Functions
Grade 12
Question:
<p>\(\displaystyle\int\frac{4x^3-7x^2+8x-4}{x^2(1+x^2)}\,dx\) equals (where \(C\) is the constant of integration)</p>
<li>\(4\ln x+\dfrac{7}{x}-4\tan^{-1}x+C\)</li>
<li>\(4\ln x-\dfrac{7}{x}+4\tan^{-1}x+C\)</li>
<li>\(4\ln x+\dfrac{7}{x}+4\tan^{-1}x+C\)</li>
<li>\(-4\ln x+\dfrac{7}{x}-4\tan^{-1}x+C\)</li>
Step-by-Step Solution
Key Concept: Partial fraction decomposition: write (4x^3-7x^2+8x-4)/(x^2(1+x^2)) = A/x + B/x^2 + (Cx+D)/(1+x^2). Solve for A, B, C, D.
Step 1: Partial Fraction Decomposition
The integrand is expressed in terms of partial fractions.
$$ \frac{4x^3-7x^2+8x-4}{x^2(1+x^2)} = \frac{A}{x}+\frac{B}{x^2}+\frac{Cx+D}{1+x^2} $$
For the given integral, the partial fraction decomposition is:
$$ \frac{4}{x}+\frac{7}{x^2}-\frac{4}{1+x^2} $$
Step 2: Integration
Integrate the decomposed expression term by term.
$$ \int\left(\frac{4}{x}+\frac{7}{x^2}-\frac{4}{1+x^2}\right)dx $$
$$ = 4\int\frac{1}{x}dx + 7\int x^{-2}dx - 4\int\frac{1}{1+x^2}dx $$
$$ = 4\ln|x| + 7\left(-\frac{1}{x}\right) - 4\arctan x + C $$
$$ = 4\ln|x| - \frac{7}{x} - 4\arctan x + C $$
Correct Answer: A