Express 2x3−4x2−5x+3(x−3)(x2−9)\frac{2x^3 - 4x^2 - 5x + 3}{(x - 3)(x^2 - 9)} in partial fractions. Hence find ∫2x3−4x2−5x+3(x−3)(x2−9) dx\int \frac{2x^3 - 4x^2 - 5x + 3}{(x - 3)(x^2 - 9)} \, dx.

Work through it with AI →

It won't just hand you the answer — it asks where you're stuck.How the checker works

Worked solution

4(i)

1. Factorise the denominator and identify that the fraction is improper.

Denominator: (x−3)(x2−9)=(x−3)(x−3)(x+3)=(x−3)2(x+3)(x - 3)(x^2 - 9) = (x - 3)(x - 3)(x + 3) = (x - 3)^2(x + 3)
Expanding the denominator gives x3−3x2−9x+27x^3 - 3x^2 - 9x + 27.
The degree of the numerator (33) is equal to the degree of the denominator (33), so it is an improper fraction.

2. Perform long division to extract the polynomial part.

2\cline2−2x3−3x2−9x+27\vline  2x3−4x2−5x+3\vline  2x3−6x2−18x+54\cline2−2\vline  2x2+13x−51\begin{array}{rl} 2 \\ \cline{2-2} x^3 - 3x^2 - 9x + 27 & \vline \; 2x^3 - 4x^2 - 5x + 3 \\ & \vline \; 2x^3 - 6x^2 - 18x + 54 \\ \cline{2-2} & \vline \; 2x^2 + 13x - 51 \end{array}

So, 2x3−4x2−5x+3(x−3)(x2−9)=2+2x2+13x−51(x−3)2(x+3)\frac{2x^3 - 4x^2 - 5x + 3}{(x - 3)(x^2 - 9)} = 2 + \frac{2x^2 + 13x - 51}{(x - 3)^2(x + 3)}.

3. Set up the partial fraction decomposition for the proper fraction part.

Let 2x2+13x−51(x−3)2(x+3)=Ax−3+B(x−3)2+Cx+3\frac{2x^2 + 13x - 51}{(x - 3)^2(x + 3)} = \frac{A}{x - 3} + \frac{B}{(x - 3)^2} + \frac{C}{x + 3}.
Multiplying by (x−3)2(x+3)(x - 3)^2(x + 3) gives:
2x2+13x−51=A(x−3)(x+3)+B(x+3)+C(x−3)22x^2 + 13x - 51 = A(x - 3)(x + 3) + B(x + 3) + C(x - 3)^2

4. Solve for the constants $A$, $B$, and $C$.

Substitute x=3x = 3:
2(3)2+13(3)−51=B(3+3)2(3)^2 + 13(3) - 51 = B(3 + 3)
18+39−51=6B  ⟹  6=6B  ⟹  B=118 + 39 - 51 = 6B \implies 6 = 6B \implies B = 1

Substitute x=−3x = -3:
2(−3)2+13(−3)−51=C(−3−3)22(-3)^2 + 13(-3) - 51 = C(-3 - 3)^2
18−39−51=36C  ⟹  −72=36C  ⟹  C=−218 - 39 - 51 = 36C \implies -72 = 36C \implies C = -2

Compare coefficients of x2x^2:
2=A+C2 = A + C
2=A−2  ⟹  A=42 = A - 2 \implies A = 4

5. Write the final expression.

2x3−4x2−5x+3(x−3)(x2−9)=2+4x−3+1(x−3)2−2x+3\frac{2x^3 - 4x^2 - 5x + 3}{(x - 3)(x^2 - 9)} = 2 + \frac{4}{x - 3} + \frac{1}{(x - 3)^2} - \frac{2}{x + 3}

Answer

2+4x−3+1(x−3)2−2x+32 + \frac{4}{x - 3} + \frac{1}{(x - 3)^2} - \frac{2}{x + 3}

4(ii)

1. Express the integrand using the partial fractions found in part (i)

From part (i), we have:
2x3−4x2−5x+3(x−3)(x2−9)=2+4x−3+1(x−3)2−2x+3\frac{2x^3 - 4x^2 - 5x + 3}{(x - 3)(x^2 - 9)} = 2 + \frac{4}{x - 3} + \frac{1}{(x - 3)^2} - \frac{2}{x + 3}

Therefore, the integral becomes:
∫(2+4x−3+1(x−3)2−2x+3) dx\int \left( 2 + \frac{4}{x - 3} + \frac{1}{(x - 3)^2} - \frac{2}{x + 3} \right) \, dx

2. Integrate each term with respect to $x$

∫2 dx+∫4x−3 dx+∫(x−3)−2 dx−∫2x+3 dx\int 2 \, dx + \int \frac{4}{x - 3} \, dx + \int (x - 3)^{-2} \, dx - \int \frac{2}{x + 3} \, dx
=2x+4ln⁡∣x−3∣+(x−3)−1−1−2ln⁡∣x+3∣+c= 2x + 4\ln|x - 3| + \frac{(x - 3)^{-1}}{-1} - 2\ln|x + 3| + c
=2x+4ln⁡∣x−3∣−1x−3−2ln⁡∣x+3∣+c= 2x + 4\ln|x - 3| - \frac{1}{x - 3} - 2\ln|x + 3| + c

Answer

2x+4ln⁡∣x−3∣−1x−3−2ln⁡∣x+3∣+c2x + 4\ln|x - 3| - \frac{1}{x - 3} - 2\ln|x + 3| + c

AI analysis — may contain mistakes.

Worked this one out on paper? Snap your steps and Gigle's AI math checker will circle the step where it went wrong.

More questions on this topic

  1. 1Express 2x4+x3+9x2+7x+3(x+1)(x2+5)\frac{2x^4 + x^3 + 9x^2 + 7x + 3}{(x+1)(x^2+5)} in partial fractions.
  2. 2Express x2+3x+2x2(2x+1)\dfrac{x^2+3x+2}{x^2(2x+1)} in partial fractions. Hence integrate x2+3x+2x2(2x+1)\dfrac{x^2+3x+2}{x^2(2x+1)} with respect t…
  3. 3Express 8x+13(1+2x)(2+x)2\frac{8x+13}{(1+2x)(2+x)^2} in partial fractions. Express 8x+13(1+2x)(2+x)2\frac{8x+13}{(1+2x)(2+x)^2} in partial fractions…
  4. 4Express 2x2+3xx2+3x+2\dfrac{2x^{2}+3x}{x^{2}+3x+2} in partial fractions. Hence evaluate ∫132x2+3xx2+3x+2 dx\displaystyle \int_{1}^{3} \dfrac{2x^{2}+3x}{x^{2}+3x+2}\,dx…
  5. 5Express 3x2+5x−2(x+1)(x2+4)\frac{3x^2 + 5x - 2}{(x + 1)(x^2 + 4)} in partial fractions.