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Resolve the following into partial fractions (a) (x-11)/(x2-x-2) (b) (3-x)/((x2+3)(x+3)) (c) (x3-6x+9)/(x2+x-2)

Mathematics

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Here's the solution for Question Q1:

Q1 a)

We need to resolve x−11x2−x−2\frac{x - 11}{x^2 - x - 2} into partial fractions.

  1. Factor the denominator.

    x2−x−2=(x−2)(x+1)x^2 - x - 2 = (x-2)(x+1)
  2. Set up the partial fraction decomposition.

    Since the denominator has distinct linear factors, we write:

    x−11(x−2)(x+1)=Ax−2+Bx+1\frac{x - 11}{(x-2)(x+1)} = \frac{A}{x-2} + \frac{B}{x+1}
  3. Combine the fractions on the right side.

    x−11(x−2)(x+1)=A(x+1)+B(x−2)(x−2)(x+1)\frac{x - 11}{(x-2)(x+1)} = \frac{A(x+1) + B(x-2)}{(x-2)(x+1)}
  4. Equate the numerators.

    x−11=A(x+1)+B(x−2)x - 11 = A(x+1) + B(x-2)
  5. Solve for A and B by substituting values for xx.

    Let x=2x = 2:

    2−11=A(2+1)+B(2−2)2 - 11 = A(2+1) + B(2-2) −9=3A+0-9 = 3A + 0 A=−3A = -3

    Let x=−1x = -1:

    −1−11=A(−1+1)+B(−1−2)-1 - 11 = A(-1+1) + B(-1-2) −12=0−3B-12 = 0 - 3B −12=−3B-12 = -3B B=4B = 4
  6. Write the partial fraction decomposition.

    x−11x2−x−2=−3x−2+4x+1\frac{x - 11}{x^2 - x - 2} = \frac{-3}{x-2} + \frac{4}{x+1}

    The partial fraction decomposition is 4x+1−3x−2\boxed{\text{}\frac{4}{x+1} - \frac{3}{x-2}\text{}}

    Q1 b)

    We need to resolve 3−x(x2+3)(x+3)\frac{3 - x}{(x^2 + 3)(x + 3)} into partial fractions.

  7. Identify the types of factors in the denominator.

    The denominator has an irreducible quadratic factor (x2+3x^2 + 3) and a linear factor (x+3x+3).

  8. Set up the partial fraction decomposition.

    3−x(x2+3)(x+3)=Ax+Bx2+3+Cx+3\frac{3 - x}{(x^2 + 3)(x + 3)} = \frac{Ax + B}{x^2 + 3} + \frac{C}{x+3}
  9. Combine the fractions on the right side.

    3−x(x2+3)(x+3)=(Ax+B)(x+3)+C(x2+3)(x2+3)(x+3)\frac{3 - x}{(x^2 + 3)(x + 3)} = \frac{(Ax + B)(x+3) + C(x^2 + 3)}{(x^2 + 3)(x + 3)}
  10. Equate the numerators.

    3−x=(Ax+B)(x+3)+C(x2+3)3 - x = (Ax + B)(x+3) + C(x^2 + 3)
  11. Solve for A, B, and C.

    Let x=−3x = -3:

    3−(−3)=(A(−3)+B)(−3+3)+C((−3)2+3)3 - (-3) = (A(-3) + B)(-3+3) + C((-3)^2 + 3) 6=0+C(9+3)6 = 0 + C(9+3) 6=12C6 = 12C C=612=12C = \frac{6}{12} = \frac{1}{2}

    Expand the equation and group terms by powers of xx:

    3−x=Ax2+3Ax+Bx+3B+Cx2+3C3 - x = Ax^2 + 3Ax + Bx + 3B + Cx^2 + 3C 3−x=(A+C)x2+(3A+B)x+(3B+3C)3 - x = (A+C)x^2 + (3A+B)x + (3B+3C)

    Compare coefficients:
    Coefficient of x2x^2: 0=A+C0 = A+C
    Since C=12C = \frac{1}{2}, then A=−12A = -\frac{1}{2}.
    Coefficient of xx: −1=3A+B-1 = 3A+B
    Substitute A=−12A = -\frac{1}{2}:

    −1=3(−12)+B-1 = 3\left(-\frac{1}{2}\right) + B −1=−32+B-1 = -\frac{3}{2} + B B=−1+32=12B = -1 + \frac{3}{2} = \frac{1}{2}

    (Check constant term: 3=3B+3C=3(12)+3(12)=32+32=62=33 = 3B+3C = 3(\frac{1}{2}) + 3(\frac{1}{2}) = \frac{3}{2} + \frac{3}{2} = \frac{6}{2} = 3. This is consistent.)

  12. Write the partial fraction decomposition.

    3−x(x2+3)(x+3)=−12x+12x2+3+12x+3\frac{3 - x}{(x^2 + 3)(x + 3)} = \frac{-\frac{1}{2}x + \frac{1}{2}}{x^2 + 3} + \frac{\frac{1}{2}}{x+3} 3−x(x2+3)(x+3)=1−x2(x2+3)+12(x+3)\frac{3 - x}{(x^2 + 3)(x + 3)} = \frac{1-x}{2(x^2 + 3)} + \frac{1}{2(x+3)}

    The partial fraction decomposition is 1−x2(x2+3)+12(x+3)\boxed{\text{}\frac{1-x}{2(x^2 + 3)} + \frac{1}{2(x+3)}\text{}}

    Q1 c)

    We need to resolve x3−6x+9x2+x−2\frac{x^3 - 6x + 9}{x^2 + x - 2} into partial fractions.

  13. Perform polynomial long division because the degree of the numerator (3) is greater than the degree of the denominator (2).

    \multicolumn2rx−1\cline2−5x2+x−2x3+0x2−6x+9\multicolumn2r−(x3+x2−2x)\cline2−4\multicolumn2r0−x2−4x+9\multicolumn2r−(−x2−x+2)\cline3−5\multicolumn2r0−3x+7\begin{array}{c|cc cc} \multicolumn{2}{r}{x} & -1 \\ \cline{2-5} x^2+x-2 & x^3 & +0x^2 & -6x & +9 \\ \multicolumn{2}{r}{-(x^3} & +x^2 & -2x) \\ \cline{2-4} \multicolumn{2}{r}{0} & -x^2 & -4x & +9 \\ \multicolumn{2}{r}{} & -(-x^2 & -x & +2) \\ \cline{3-5} \multicolumn{2}{r}{} & 0 & -3x & +7 \\ \end{array}

    So, x3−6x+9x2+x−2=x−1+−3x+7x2+x−2\frac{x^3 - 6x + 9}{x^2 + x - 2} = x - 1 + \frac{-3x + 7}{x^2 + x - 2}.

  14. Factor the denominator of the remainder term.

    x2+x−2=(x+2)(x−1)x^2 + x - 2 = (x+2)(x-1)
  15. Set up the partial fraction decomposition for the remainder term.

    −3x+7(x+2)(x−1)=Ax+2+Bx−1\frac{-3x + 7}{(x+2)(x-1)} = \frac{A}{x+2} + \frac{B}{x-1}
  16. Combine the fractions on the right side.

    −3x+7(x+2)(x−1)=A(x−1)+B(x+2)(x+2)(x−1)\frac{-3x + 7}{(x+2)(x-1)} = \frac{A(x-1) + B(x+2)}{(x+2)(x-1)}
  17. Equate the numerators.

    −3x+7=A(x−1)+B(x+2)-3x + 7 = A(x-1) + B(x+2)
  18. Solve for A and B by substituting values for xx.

    Let x=1x = 1:

    −3(1)+7=A(1−1)+B(1+2)-3(1) + 7 = A(1-1) + B(1+2) 4=0+3B4 = 0 + 3B B=43B = \frac{4}{3}

    Let x=−2x = -2:

    −3(−2)+7=A(−2−1)+B(−2+2)-3(-2) + 7 = A(-2-1) + B(-2+2) 6+7=−3A+06 + 7 = -3A + 0 13=−3A13 = -3A A=−133A = -\frac{13}{3}
  19. Write the complete partial fraction decomposition.

    x3−6x+9x2+x−2=x−1+−133x+2+43x−1\frac{x^3 - 6x + 9}{x^2 + x - 2} = x - 1 + \frac{-\frac{13}{3}}{x+2} + \frac{\frac{4}{3}}{x-1} x3−6x+9x2+x−2=x−1−133(x+2)+43(x−1)\frac{x^3 - 6x + 9}{x^2 + x - 2} = x - 1 - \frac{13}{3(x+2)} + \frac{4}{3(x-1)}

    The partial fraction decomposition is x−1−133(x+2)+43(x−1)\boxed{\text{}x - 1 - \frac{13}{3(x+2)} + \frac{4}{3(x-1)}\text{}}

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