将 3x2−4x2(3x−2)\frac{3x^2 - 4}{x^2(3x - 2)} 表示为部分分式。 由此,求 ∫3x2−4x2(3x−2) dx\int \frac{3x^2 - 4}{x^2(3x - 2)} \, dx 的值。

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完整解法

3(a)

1. State the correct form for the partial fractions

Let 3x2−4x2(3x−2)=Ax+Bx2+C3x−2\frac{3x^2 - 4}{x^2(3x - 2)} = \frac{A}{x} + \frac{B}{x^2} + \frac{C}{3x - 2}

2. Form the basic equation by clearing the denominator

3x2−4=Ax(3x−2)+B(3x−2)+Cx23x^2 - 4 = Ax(3x - 2) + B(3x - 2) + Cx^2

3. Find the values of $B$ and $C$ by substituting suitable values of $x$

Substitute x=0x = 0:
−4=B(−2)  ⟹  B=2-4 = B(-2) \implies B = 2

Substitute x=23x = \frac{2}{3}:
3(23)2−4=C(23)23\left(\frac{2}{3}\right)^2 - 4 = C\left(\frac{2}{3}\right)^2
43−4=49C\frac{4}{3} - 4 = \frac{4}{9}C
−83=49C  ⟹  C=−6-\frac{8}{3} = \frac{4}{9}C \implies C = -6

4. Find the value of $A$ by comparing coefficients

Compare coefficients of x2x^2:
3=3A+C3 = 3A + C
3=3A−63 = 3A - 6
3A=9  ⟹  A=33A = 9 \implies A = 3

答案

3x+2x2−63x−2\frac{3}{x} + \frac{2}{x^2} - \frac{6}{3x - 2}

3(b)

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

From part (a), we first find the partial fractions for 3x2−4x2(3x−2)\frac{3x^2 - 4}{x^2(3x - 2)}.
Let 3x2−4x2(3x−2)=Ax+Bx2+C3x−2\frac{3x^2 - 4}{x^2(3x - 2)} = \frac{A}{x} + \frac{B}{x^2} + \frac{C}{3x - 2}.
3x2−4=Ax(3x−2)+B(3x−2)+Cx23x^2 - 4 = Ax(3x - 2) + B(3x - 2) + Cx^2
Substituting x=0x = 0 gives −4=−2B  ⟹  B=2-4 = -2B \implies B = 2.
Substituting x=23x = \frac{2}{3} gives 3(49)−4=C(49)  ⟹  −83=49C  ⟹  C=−63(\frac{4}{9}) - 4 = C(\frac{4}{9}) \implies -\frac{8}{3} = \frac{4}{9}C \implies C = -6.
Comparing coefficients of x2x^2 gives 3=3A+C  ⟹  3=3A−6  ⟹  A=33 = 3A + C \implies 3 = 3A - 6 \implies A = 3.
Thus, 3x2−4x2(3x−2)=3x+2x2−63x−2\frac{3x^2 - 4}{x^2(3x - 2)} = \frac{3}{x} + \frac{2}{x^2} - \frac{6}{3x - 2}.

∫3x2−4x2(3x−2) dx=∫(3x+2x−2−63x−2) dx\int \frac{3x^2 - 4}{x^2(3x - 2)} \, dx = \int \left( \frac{3}{x} + 2x^{-2} - \frac{6}{3x - 2} \right) \, dx

2. Integrate each term using standard integration rules

=3ln⁡x+2x−1−1−6ln⁡(3x−2)3+c= 3 \ln x + \frac{2x^{-1}}{-1} - \frac{6 \ln(3x - 2)}{3} + c
=3ln⁡x−2x−2ln⁡(3x−2)+c= 3 \ln x - \frac{2}{x} - 2 \ln(3x - 2) + c

答案

3ln⁡x−2x−2ln⁡(3x−2)+c3 \ln x - \frac{2}{x} - 2 \ln(3x - 2) + c

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