Express 2x3−4x2−5x+3(x−3)(x2−9)\frac{2x^3 - 4x^2 - 5x + 3}{(x-3)(x^2-9)} in partial fractions.

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Worked solution

4(i)

1. Expand the denominator and perform long division since the degree of the numerator equals the degree of the denominator.

Denominator: (x−3)(x2−9)=(x−3)(x−3)(x+3)=x3−3x2−9x+27(x-3)(x^2-9) = (x-3)(x-3)(x+3) = x^3 - 3x^2 - 9x + 27

Since the numerator 2x3−4x2−5x+32x^3 - 4x^2 - 5x + 3 has degree 3, we divide it by the denominator:
2x3−4x2−5x+3x3−3x2−9x+27=2+(2x3−4x2−5x+3)−2(x3−3x2−9x+27)(x−3)2(x+3)\frac{2x^3 - 4x^2 - 5x + 3}{x^3 - 3x^2 - 9x + 27} = 2 + \frac{(2x^3 - 4x^2 - 5x + 3) - 2(x^3 - 3x^2 - 9x + 27)}{(x-3)^2(x+3)}
=2+2x2+13x−51(x−3)2(x+3)= 2 + \frac{2x^2 + 13x - 51}{(x-3)^2(x+3)}

2. 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

3. 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

4. Write the final expression in partial fractions.

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}

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