By writing x=A(8x−8)+Bx = A(8x - 8) + B, where AA and BB are constants, find ∫01x4x2−8x+5 dx\int_{0}^{1} \frac{x}{4x^2 - 8x + 5} \,\text{d}x, giving your answer in the form pln⁡q+rtan⁡−1sp \ln q + r \tan^{-1} s, where pp, qq, rr and ss are values to be determined.

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Worked solution

4

1. Determine the constants $A$ and $B$ by comparing coefficients.

x=A(8x−8)+Bx = A(8x - 8) + B
x=8Ax−8A+Bx = 8Ax - 8A + B
Comparing the coefficients of xx:
8A=1  ⟹  A=188A = 1 \implies A = \frac{1}{8}
Comparing the constant terms:
−8A+B=0-8A + B = 0
−8(18)+B=0  ⟹  B=1-8\left(\frac{1}{8}\right) + B = 0 \implies B = 1

2. Rewrite the integral by splitting it into two parts using the partial numerator.

Substitute AA and BB back into the integrand:
∫01x4x2−8x+5 dx=∫0118(8x−8)+14x2−8x+5 dx\int_{0}^{1} \frac{x}{4x^2 - 8x + 5} \,\text{d}x = \int_{0}^{1} \frac{\frac{1}{8}(8x - 8) + 1}{4x^2 - 8x + 5} \,\text{d}x
=18∫018x−84x2−8x+5 dx+∫0114x2−8x+5 dx= \frac{1}{8} \int_{0}^{1} \frac{8x - 8}{4x^2 - 8x + 5} \,\text{d}x + \int_{0}^{1} \frac{1}{4x^2 - 8x + 5} \,\text{d}x

3. Evaluate the first integral using the standard form $\int \frac{\mathrm{f}'(x)}{\mathrm{f}(x)} \,\text{d}x$.

18∫018x−84x2−8x+5 dx=18[ln⁡∣4x2−8x+5∣]01\frac{1}{8} \int_{0}^{1} \frac{8x - 8}{4x^2 - 8x + 5} \,\text{d}x = \frac{1}{8} \left[ \ln |4x^2 - 8x + 5| \right]_0^1
=18(ln⁡(4(1)2−8(1)+5)−ln⁡(4(0)2−8(0)+5))= \frac{1}{8} \left( \ln(4(1)^2 - 8(1) + 5) - \ln(4(0)^2 - 8(0) + 5) \right)
=18(ln⁡1−ln⁡5)=−18ln⁡5= \frac{1}{8} (\ln 1 - \ln 5) = -\frac{1}{8} \ln 5

4. Complete the square for the denominator of the second integral.

4x2−8x+5=4(x2−2x)+54x^2 - 8x + 5 = 4(x^2 - 2x) + 5
=4(x2−2x+1)−4+5= 4(x^2 - 2x + 1) - 4 + 5
=4(x−1)2+1= 4(x - 1)^2 + 1

5. Evaluate the second integral using the standard form $\int \frac{1}{x^2 + a^2} \,\text{d}x$ and combine.

∫0114(x−1)2+1 dx=14∫011(x−1)2+(12)2 dx\int_{0}^{1} \frac{1}{4(x - 1)^2 + 1} \,\text{d}x = \frac{1}{4} \int_{0}^{1} \frac{1}{(x - 1)^2 + \left(\frac{1}{2}\right)^2} \,\text{d}x
=14[112tan⁡−1(x−112)]01=12[tan⁡−1(2x−2)]01= \frac{1}{4} \left[ \frac{1}{\frac{1}{2}} \tan^{-1} \left( \frac{x - 1}{\frac{1}{2}} \right) \right]_0^1 = \frac{1}{2} \left[ \tan^{-1}(2x - 2) \right]_0^1
=12(tan⁡−1(0)−tan⁡−1(−2))= \frac{1}{2} (\tan^{-1}(0) - \tan^{-1}(-2))
Since tan⁡−1(−x)=−tan⁡−1(x)\tan^{-1}(-x) = -\tan^{-1}(x), we have:
=12(0−(−tan⁡−12))=12tan⁡−12= \frac{1}{2} (0 - (-\tan^{-1} 2)) = \frac{1}{2} \tan^{-1} 2

Combining both parts:
∫01x4x2−8x+5 dx=−18ln⁡5+12tan⁡−12\int_{0}^{1} \frac{x}{4x^2 - 8x + 5} \,\text{d}x = -\frac{1}{8} \ln 5 + \frac{1}{2} \tan^{-1} 2

Answer

-\frac{1}{8} \ln 5 + \frac{1}{2} \tan^{-1} 2

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