Express 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 to xx.

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Worked solution

4(a)

1. Set up the partial fraction decomposition form for a repeated linear factor and a distinct linear factor

Let x2+3x+2x2(2x+1)=Ax+Bx2+C2x+1\dfrac{x^2+3x+2}{x^2(2x+1)} = \dfrac{A}{x} + \dfrac{B}{x^2} + \dfrac{C}{2x+1}

2. Multiply by the common denominator to form an identity

x2+3x+2=Ax(2x+1)+B(2x+1)+Cx2x^2+3x+2 = Ax(2x+1) + B(2x+1) + Cx^2

3. Substitute $x = 0$ to find $B$

Let x=0x = 0:
02+3(0)+2=A(0)+B(2(0)+1)+C(0)0^2 + 3(0) + 2 = A(0) + B(2(0)+1) + C(0)
2=B(1)2 = B(1)
B=2B = 2

4. Substitute $x = -\frac{1}{2}$ to find $C$

Let x=−12x = -\frac{1}{2}:
(−12)2+3(−12)+2=C(−12)2(-\frac{1}{2})^2 + 3(-\frac{1}{2}) + 2 = C(-\frac{1}{2})^2
14−32+2=14C\frac{1}{4} - \frac{3}{2} + 2 = \frac{1}{4}C
34=14C\frac{3}{4} = \frac{1}{4}C
C=3C = 3

5. Compare coefficients of $x^2$ to find $A$

Comparing coefficients of x2x^2:
1=2A+C1 = 2A + C
1=2A+31 = 2A + 3
2A=−22A = -2
A=−1A = -1

Therefore, x2+3x+2x2(2x+1)=−1x+2x2+32x+1\dfrac{x^2+3x+2}{x^2(2x+1)} = -\dfrac{1}{x} + \dfrac{2}{x^2} + \dfrac{3}{2x+1}

Answer

−1x+2x2+32x+1-\frac{1}{x} + \frac{2}{x^2} + \frac{3}{2x+1}

4(b)

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

From part (a), we have:
x2+3x+2x2(2x+1)=−1x+2x2+32x+1\dfrac{x^2+3x+2}{x^2(2x+1)} = -\dfrac{1}{x} + \dfrac{2}{x^2} + \dfrac{3}{2x+1}

2. Integrate each term with respect to $x$

∫(−1x+2x−2+32x+1)dx\int \left( -\dfrac{1}{x} + 2x^{-2} + \dfrac{3}{2x+1} \right) dx
=−ln⁡x+2x−1−1+32ln⁡(2x+1)+c= -\ln x + \dfrac{2x^{-1}}{-1} + \dfrac{3}{2}\ln(2x+1) + c
=−ln⁡x−2x+32ln⁡(2x+1)+c= -\ln x - \dfrac{2}{x} + \dfrac{3}{2}\ln(2x+1) + c

Answer

−ln⁡x−2x+32ln⁡(2x+1)+c-\ln x - \frac{2}{x} + \frac{3}{2}\ln(2x+1) + c

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