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Worked solution

Integrate partial fractions with logarithm and arctangent terms

Using partial fractions, find ∫029x2+x−13(2x−5)(x2+9) dx\displaystyle\int_0^2\frac{9x^2+x-13}{(2x-5)(x^2+9)}\,dx. Give your answer as aln⁡b+ctan⁡−1da\ln b+c\tan^{-1}d, with rational a,b,c,da,b,c,d.

[2014/II/2]

CJC · Discussion · Question 2

Answer

32ln⁡(13/45)+83tan⁡−1(2/3)\tfrac32\ln(13/45)+\tfrac83\tan^{-1}(2/3).

Find the partial-fraction coefficients

Write the fraction as A/(2x−5)+(Bx+C)/(x2+9)A/(2x-5)+(Bx+C)/(x^2+9). Then 9x2+x−13=A(x2+9)+(Bx+C)(2x−5)9x^2+x-13=A(x^2+9)+(Bx+C)(2x-5). At x=5/2x=5/2, A=3A=3; at x=0x=0, −13=27−5C-13=27-5C, so C=8C=8; at x=1x=1, −3=30−3(B+8)-3=30-3(B+8), so B=3B=3. Thus the integrand is 3/(2x−5)+(3x+8)/(x2+9)3/(2x-5)+(3x+8)/(x^2+9).

How did this part go?

Integrate and evaluate the endpoints

An antiderivative is 32ln⁡∣2x−5∣+32ln⁡(x2+9)+83tan⁡−1(x/3)\tfrac32\ln|2x-5|+\tfrac32\ln(x^2+9)+\tfrac83\tan^{-1}(x/3). Evaluation from 00 to 22 gives 32(ln⁡1−ln⁡5+ln⁡13−ln⁡9)+83tan⁡−1(2/3)=32ln⁡(13/45)+83tan⁡−1(2/3)\tfrac32(\ln1-\ln5+\ln13-\ln9)+\tfrac83\tan^{-1}(2/3)=\tfrac32\ln(13/45)+\tfrac83\tan^{-1}(2/3). Hence a=3/2a=3/2, b=13/45b=13/45, c=8/3c=8/3, d=2/3d=2/3.

How did this part go?

Common mistakes

Use ln|2x − 5| because 2x − 5 is negative throughout the integration interval.

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