Part of CALC-10 — Integration: Advanced Techniques & Reduction

Partial Fraction Decomposition

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Step 1: Ensure proper fraction. If deg(P) >= deg(Q), perform polynomial long division: P/Q = quotient + remainder/Q.

Step 2: Factor the denominator Q(x). Over the reals, Q factors into products of linear factors (x-a)^k and irreducible quadratic factors (x2+px+qx^{2+px+q})^m where p24qp^{2-4q} < 0.

Step 3: Write the decomposition form.

  • Linear (x-a): Axa\frac{A}{x-a}
  • Repeated linear (x-a)^k: A1xa\frac{A1}{x-a} + A2xa\frac{A2}{x-a}^2 + ... + Akxa\frac{Ak}{x-a}^k
  • Quadratic (x2+px+qx^{2+px+q}): Ax+B(x2+px+q)\frac{Ax+B}{(x^2+px+q)}
  • Repeated quadratic: stack with increasing powers in denominator

Step 4: Find coefficients.

  • Cover-up: For distinct linear factor (x-a), set x=a after removing (x-a). Quick and efficient.
  • Substitution: Plug in convenient x values (usually roots of Q).
  • Coefficient comparison: Match coefficients of powers of x on both sides.

Step 5: Integrate each term.

  • Axa\frac{A}{x-a} => A*ln|x-a|
  • Axa\frac{A}{x-a}^n (n>1) => A*(x-a)^1n(1n)\frac{1-n}{(1-n)}
  • Ax+B(x2+px+q)\frac{Ax+B}{(x^2+px+q)}: write Ax+B = A2\frac{A}{2}(2x+p) + (B-Ap/2). First part gives log, second gives arctan after completing the square.

Common JEE types:

  • 1(xa\frac{1}{(x-a}(x-b)): two distinct linear factors
  • 1x(xn+1\frac{1}{x(x^n+1}): multiply by x^n1x\frac{n-1}{x}^(n-1), then substitute t = xnx^n
  • px+q((x2+a)\frac{px+q}{((x^2+a)}(x2+bx^{2+b})): two quadratic factors

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