Express 9x2+17x−163x2+5x−2\frac{9x^2 + 17x - 16}{3x^2 + 5x - 2} in partial fractions. Hence, find ∫9x2+17x−163x2+5x−2 dx\int \frac{9x^2 + 17x - 16}{3x^2 + 5x - 2} \, dx.

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Worked solution

3(a)

1. Perform long division to extract the polynomial part since the degree of the numerator equals the degree of the denominator.

9x2+17x−163x2+5x−2=3(3x2+5x−2)−15x+6+17x−163x2+5x−2\frac{9x^2 + 17x - 16}{3x^2 + 5x - 2} = \frac{3(3x^2 + 5x - 2) - 15x + 6 + 17x - 16}{3x^2 + 5x - 2}
=3+2x−103x2+5x−2= 3 + \frac{2x - 10}{3x^2 + 5x - 2}

2. Factorise the denominator of the proper fraction.

3x2+5x−2=(3x−1)(x+2)3x^2 + 5x - 2 = (3x - 1)(x + 2)

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

Let 2x−10(3x−1)(x+2)=A3x−1+Bx+2\frac{2x - 10}{(3x - 1)(x + 2)} = \frac{A}{3x - 1} + \frac{B}{x + 2}
2x−10=A(x+2)+B(3x−1)2x - 10 = A(x + 2) + B(3x - 1)

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

Substitute x=−2x = -2:
2(−2)−10=B(3(−2)−1)2(-2) - 10 = B(3(-2) - 1)
−14=−7B  ⟹  B=2-14 = -7B \implies B = 2

Substitute x=13x = \frac{1}{3}:
2(13)−10=A(13+2)2\left(\frac{1}{3}\right) - 10 = A\left(\frac{1}{3} + 2\right)
−283=73A  ⟹  A=−4-\frac{28}{3} = \frac{7}{3}A \implies A = -4

5. Write the final expression in partial fractions.

9x2+17x−163x2+5x−2=3−43x−1+2x+2\frac{9x^2 + 17x - 16}{3x^2 + 5x - 2} = 3 - \frac{4}{3x - 1} + \frac{2}{x + 2}

Answer

3−43x−1+2x+23 - \frac{4}{3x - 1} + \frac{2}{x + 2}

3(b)

1. Use the partial fraction expansion from part (a)

From part (a), we have:
9x2+17x−163x2+5x−2=3−43x−1+2x+2\frac{9x^2 + 17x - 16}{3x^2 + 5x - 2} = 3 - \frac{4}{3x - 1} + \frac{2}{x + 2}

2. Integrate each term with respect to $x$

∫(3−43x−1+2x+2) dx\int \left( 3 - \frac{4}{3x - 1} + \frac{2}{x + 2} \right) \, dx
=3x−43ln⁡(3x−1)+2ln⁡(x+2)+C= 3x - \frac{4}{3}\ln(3x - 1) + 2\ln(x + 2) + C

Answer

3x−43ln⁡(3x−1)+2ln⁡(x+2)+C3x - \frac{4}{3}\ln(3x - 1) + 2\ln(x + 2) + C

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