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Manipulate the numerator to match the derivative of the denominator.

Mathematics

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To find the integral ∫2x3x2+4x+2dx\int \frac{2x}{3x^2+4x+2} dx, we will use a combination of substitution and completing the square.

  1. Manipulate the numerator to match the derivative of the denominator.

    Let the denominator be f(x)=3x2+4x+2f(x) = 3x^2+4x+2. Its derivative is f′(x)=6x+4f'(x) = 6x+4.
    We want to express the numerator 2x2x in terms of 6x+46x+4.
    We can write 2x=A(6x+4)+B2x = A(6x+4) + B.
    Comparing coefficients:
    For xx: 2=6A  ⟹  A=132 = 6A \implies A = \frac{1}{3}.
    For constants: 0=4A+B  ⟹  0=4(13)+B  ⟹  B=−430 = 4A + B \implies 0 = 4\left(\frac{1}{3}\right) + B \implies B = -\frac{4}{3}.
    So, 2x=13(6x+4)−432x = \frac{1}{3}(6x+4) - \frac{4}{3}.

  2. Split the integral into two parts.

    ∫2x3x2+4x+2dx=∫13(6x+4)−433x2+4x+2dx\int \frac{2x}{3x^2+4x+2} dx = \int \frac{\frac{1}{3}(6x+4) - \frac{4}{3}}{3x^2+4x+2} dx =13∫6x+43x2+4x+2dx−43∫13x2+4x+2dx= \frac{1}{3} \int \frac{6x+4}{3x^2+4x+2} dx - \frac{4}{3} \int \frac{1}{3x^2+4x+2} dx
  3. Evaluate the first integral.

    For the first integral, let u=3x2+4x+2u = 3x^2+4x+2. Then du=(6x+4)dxdu = (6x+4) dx.

    13∫duu=13ln⁡∣u∣+C1=13ln⁡(3x2+4x+2)+C1\frac{1}{3} \int \frac{du}{u} = \frac{1}{3} \ln|u| + C_1 = \frac{1}{3} \ln(3x^2+4x+2) + C_1

    (Note: 3x2+4x+23x^2+4x+2 is always positive because its discriminant 42−4(3)(2)=16−24=−8<04^2 - 4(3)(2) = 16-24 = -8 < 0 and the leading coefficient is positive.)

  4. Evaluate the second integral by completing the square in the denominator.

    For the second integral, consider the denominator 3x2+4x+23x^2+4x+2.

    3x2+4x+2=3(x2+43x+23)3x^2+4x+2 = 3\left(x^2 + \frac{4}{3}x + \frac{2}{3}\right)

    Complete the square for x2+43xx^2 + \frac{4}{3}x:

    x2+43x+(43⋅2)2−(43⋅2)2+23x^2 + \frac{4}{3}x + \left(\frac{4}{3 \cdot 2}\right)^2 - \left(\frac{4}{3 \cdot 2}\right)^2 + \frac{2}{3} =(x+23)2−49+23=(x+23)2−49+69=(x+23)2+29= \left(x + \frac{2}{3}\right)^2 - \frac{4}{9} + \frac{2}{3} = \left(x + \frac{2}{3}\right)^2 - \frac{4}{9} + \frac{6}{9} = \left(x + \frac{2}{3}\right)^2 + \frac{2}{9}

    So, 3x2+4x+2=3[(x+23)2+29]3x^2+4x+2 = 3\left[\left(x + \frac{2}{3}\right)^2 + \frac{2}{9}\right].
    The second integral becomes:

    −43∫13[(x+23)2+29]dx=−49∫1(x+23)2+(23)2dx-\frac{4}{3} \int \frac{1}{3\left[\left(x + \frac{2}{3}\right)^2 + \frac{2}{9}\right]} dx = -\frac{4}{9} \int \frac{1}{\left(x + \frac{2}{3}\right)^2 + \left(\frac{\sqrt{2}}{3}\right)^2} dx
  5. Apply the arctan integration formula.

    This integral is of the form ∫1u2+a2du=1aarctan⁡(ua)+C\int \frac{1}{u^2+a^2} du = \frac{1}{a} \arctan\left(\frac{u}{a}\right) + C.
    Here, u=x+23u = x + \frac{2}{3} (so du=dxdu = dx) and a=23a = \frac{\sqrt{2}}{3}.

    −49[123arctan⁡(x+2323)]+C2-\frac{4}{9} \left[ \frac{1}{\frac{\sqrt{2}}{3}} \arctan\left(\frac{x + \frac{2}{3}}{\frac{\sqrt{2}}{3}}\right) \right] + C_2 =−49[32arctan⁡(3x+2323)]+C2= -\frac{4}{9} \left[ \frac{3}{\sqrt{2}} \arctan\left(\frac{\frac{3x+2}{3}}{\frac{\sqrt{2}}{3}}\right) \right] + C_2 =−432arctan⁡(3x+22)+C2= -\frac{4}{3\sqrt{2}} \arctan\left(\frac{3x+2}{\sqrt{2}}\right) + C_2

    Rationalize the denominator:

    −423⋅2arctan⁡(3x+22)+C2=−223arctan⁡(3x+22)+C2-\frac{4\sqrt{2}}{3 \cdot 2} \arctan\left(\frac{3x+2}{\sqrt{2}}\right) + C_2 = -\frac{2\sqrt{2}}{3} \arctan\left(\frac{3x+2}{\sqrt{2}}\right) + C_2
  6. Combine the results from both integrals.

    ∫2x3x2+4x+2dx=13ln⁡(3x2+4x+2)−223arctan⁡(3x+22)+C\int \frac{2x}{3x^2+4x+2} dx = \frac{1}{3} \ln(3x^2+4x+2) - \frac{2\sqrt{2}}{3} \arctan\left(\frac{3x+2}{\sqrt{2}}\right) + C

    The final answer is 13ln⁡(3x2+4x+2)−223arctan⁡(3x+22)+C\boxed{\frac{1}{3} \ln(3x^2+4x+2) - \frac{2\sqrt{2}}{3} \arctan\left(\frac{3x+2}{\sqrt{2}}\right) + C}.

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