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Hence, show that integral_0¹ (2x+1)/((x-2)(x²+1)) dx = -(1)/(2) ln 8.

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integral01(2x+1)/((x−2)(x2+1))dx=−(1)/(2)ln8integral_0¹ (2x+1)/((x-2)(x²+1)) dx = -(1)/(2) ln 8

Step-by-step solution

4 steps

To show that ∫012x+1(x−2)(x2+1)dx=−12ln⁡8\int_{0}^{1} \frac{2x+1}{(x-2)(x^2+1)} dx = -\frac{1}{2} \ln 8, we will use partial fraction decomposition and then evaluate the definite integral.

  1. Decompose the integrand into partial fractions.

    Let 2x+1(x−2)(x2+1)=Ax−2+Bx+Cx2+1\frac{2x+1}{(x-2)(x^2+1)} = \frac{A}{x-2} + \frac{Bx+C}{x^2+1}.
    Multiply both sides by (x−2)(x2+1)(x-2)(x^2+1):

    2x+1=A(x2+1)+(Bx+C)(x−2)2x+1 = A(x^2+1) + (Bx+C)(x-2)

    To find AA, set x=2x=2:

    2(2)+1=A(22+1)+(B(2)+C)(2−2)2(2)+1 = A(2^2+1) + (B(2)+C)(2-2) 5=A(4+1)+05 = A(4+1) + 0 5=5A  ⟹  A=15 = 5A \implies A = 1

    Substitute A=1A=1 back into the equation:

    2x+1=1(x2+1)+(Bx+C)(x−2)2x+1 = 1(x^2+1) + (Bx+C)(x-2) 2x+1=x2+1+Bx2−2Bx+Cx−2C2x+1 = x^2+1 + Bx^2 - 2Bx + Cx - 2C 2x+1=(1+B)x2+(−2B+C)x+(1−2C)2x+1 = (1+B)x^2 + (-2B+C)x + (1-2C)

    Compare the coefficients of powers of xx:
    For x2x^2: 0=1+B  ⟹  B=−10 = 1+B \implies B = -1
    For xx: 2=−2B+C2 = -2B+C
    Substitute B=−1B=-1: 2=−2(−1)+C  ⟹  2=2+C  ⟹  C=02 = -2(-1)+C \implies 2 = 2+C \implies C = 0
    For the constant term: 1=1−2C1 = 1-2C
    Substitute C=0C=0: 1=1−2(0)  ⟹  1=11 = 1-2(0) \implies 1 = 1 (consistent)

    So, the partial fraction decomposition is:

    2x+1(x−2)(x2+1)=1x−2+−xx2+1=1x−2−xx2+1\frac{2x+1}{(x-2)(x^2+1)} = \frac{1}{x-2} + \frac{-x}{x^2+1} = \frac{1}{x-2} - \frac{x}{x^2+1}
  2. Integrate each term.

    We need to evaluate ∫01(1x−2−xx2+1)dx\int_{0}^{1} \left( \frac{1}{x-2} - \frac{x}{x^2+1} \right) dx.

    The integral of the first term is:

    ∫1x−2dx=ln⁡∣x−2∣\int \frac{1}{x-2} dx = \ln|x-2|

    For the second term, let u=x2+1u = x^2+1, so du=2x dx  ⟹  x dx=12dudu = 2x \, dx \implies x \, dx = \frac{1}{2} du:

    ∫xx2+1dx=∫1u⋅12du=12ln⁡∣u∣=12ln⁡(x2+1)\int \frac{x}{x^2+1} dx = \int \frac{1}{u} \cdot \frac{1}{2} du = \frac{1}{2} \ln|u| = \frac{1}{2} \ln(x^2+1)

    (Since x2+1x^2+1 is always positive, we can remove the absolute value.)

    Combining these, the indefinite integral is:

    ∫2x+1(x−2)(x2+1)dx=ln⁡∣x−2∣−12ln⁡(x2+1)\int \frac{2x+1}{(x-2)(x^2+1)} dx = \ln|x-2| - \frac{1}{2} \ln(x^2+1)
  3. Evaluate the definite integral from 0 to 1.

    [ln⁡∣x−2∣−12ln⁡(x2+1)]01\left[ \ln|x-2| - \frac{1}{2} \ln(x^2+1) \right]_{0}^{1}

    Evaluate at the upper limit x=1x=1:

    ln⁡∣1−2∣−12ln⁡(12+1)=ln⁡∣−1∣−12ln⁡(2)=ln⁡(1)−12ln⁡(2)=0−12ln⁡(2)=−12ln⁡(2)\ln|1-2| - \frac{1}{2} \ln(1^2+1) = \ln|-1| - \frac{1}{2} \ln(2) = \ln(1) - \frac{1}{2} \ln(2) = 0 - \frac{1}{2} \ln(2) = -\frac{1}{2} \ln(2)

    Evaluate at the lower limit x=0x=0:

    ln⁡∣0−2∣−12ln⁡(02+1)=ln⁡∣−2∣−12ln⁡(1)=ln⁡(2)−12(0)=ln⁡(2)\ln|0-2| - \frac{1}{2} \ln(0^2+1) = \ln|-2| - \frac{1}{2} \ln(1) = \ln(2) - \frac{1}{2}(0) = \ln(2)

    Subtract the lower limit value from the upper limit value:

    (−12ln⁡(2))−(ln⁡(2))=−12ln⁡(2)−22ln⁡(2)=−32ln⁡(2)\left( -\frac{1}{2} \ln(2) \right) - \left( \ln(2) \right) = -\frac{1}{2} \ln(2) - \frac{2}{2} \ln(2) = -\frac{3}{2} \ln(2)
  4. Show that the result matches the target value.

    We need to show that −32ln⁡(2)-\frac{3}{2} \ln(2) is equal to −12ln⁡8-\frac{1}{2} \ln 8.
    Using logarithm properties, ln⁡(ab)=bln⁡a\ln(a^b) = b \ln a:

    −12ln⁡8=−12ln⁡(23)=−12⋅3ln⁡2=−32ln⁡2-\frac{1}{2} \ln 8 = -\frac{1}{2} \ln (2^3) = -\frac{1}{2} \cdot 3 \ln 2 = -\frac{3}{2} \ln 2

    Since −32ln⁡2=−32ln⁡2-\frac{3}{2} \ln 2 = -\frac{3}{2} \ln 2, the equality holds.

    Therefore, we have shown that:

    ∫012x+1(x−2)(x2+1)dx=−12ln⁡8\int_{0}^{1} \frac{2x+1}{(x-2)(x^2+1)} dx = \boxed{-\frac{1}{2} \ln 8}
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