Express 2x3−5x2(x−1)\frac{2x^3 - 5}{x^2(x - 1)} in partial fractions.

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Worked solution

3

1. Recognise that the fraction is improper and perform long division to extract the polynomial part.

The degree of the numerator (2x3−52x^3 - 5) is 3, and the degree of the denominator (x3−x2x^3 - x^2) is also 3. Thus, the fraction is improper.
2x3−5x3−x2=2+2x2−5x2(x−1)\frac{2x^3 - 5}{x^3 - x^2} = 2 + \frac{2x^2 - 5}{x^2(x - 1)}

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

Let 2x2−5x2(x−1)=Ax+Bx2+Cx−1\frac{2x^2 - 5}{x^2(x - 1)} = \frac{A}{x} + \frac{B}{x^2} + \frac{C}{x - 1}
Multiplying throughout by x2(x−1)x^2(x - 1):
2x2−5=Ax(x−1)+B(x−1)+Cx22x^2 - 5 = Ax(x - 1) + B(x - 1) + Cx^2

3. Solve for the constants $B$ and $C$ by substituting suitable values of $x$.

Substitute x=0x = 0:
2(0)2−5=A(0)(−1)+B(−1)+C(0)22(0)^2 - 5 = A(0)(-1) + B(-1) + C(0)^2
−5=−B  ⟹  B=5-5 = -B \implies B = 5

Substitute x=1x = 1:
2(1)2−5=A(1)(0)+B(0)+C(1)22(1)^2 - 5 = A(1)(0) + B(0) + C(1)^2
−3=C  ⟹  C=−3-3 = C \implies C = -3

4. Solve for the constant $A$ by comparing coefficients.

Compare the coefficients of x2x^2 on both sides:
2=A+C2 = A + C
Since C=−3C = -3:
2=A−3  ⟹  A=52 = A - 3 \implies A = 5

5. Write the final expression in partial fractions.

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

Answer

2+5x+5x2−3x−12 + \frac{5}{x} + \frac{5}{x^2} - \frac{3}{x - 1}

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