Improper Integral: Infinite limits

Calculus Level 2

Evaluate the improper integral 0 4 x 30 ( x 2 + 5 ) ( 3 x + 2 ) d x \int_0^\infty\frac{4x-30}{(x^2+5)(3x+2)}dx


The answer is -2.4203681.

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1 solution

Firstly, 4 x 30 ( x 2 + 5 ) ( 3 x + 2 ) A x + B x 2 + 5 + C 3 x + 2 ( A x + B ) ( 3 x + 2 ) + C ( x 2 + 5 ) ( x 2 + 5 ) ( 3 x + 2 ) ( 3 A + C ) x 2 + ( 2 A + 3 B ) x + 2 B + 5 C ( x 2 + 5 ) ( 3 x + 2 ) \begin{aligned}\frac{4x-30}{(x^2+5)(3x+2)}&\equiv\frac{Ax+B}{x^2+5}+\frac{C}{3x+2}\\&\equiv \frac{(Ax+B)(3x+2)+C(x^2+5)}{(x^2+5)(3x+2)}\\&\equiv \frac{(3A+C)x^2+(2A+3B)x+2B+5C}{(x^2+5)(3x+2)}\end{aligned}

Comparing coefficients, we get A = 2 , B = 0 A=2,B=0 and C = 6 C=-6 .

0 4 x 30 ( x 2 + 5 ) ( 3 x + 2 ) d x = 0 2 x x 2 + 5 6 3 x + 2 d x = lim n 0 n 2 x x 2 + 5 6 3 x + 2 d x = lim n [ ln x 2 + 5 2 ln 3 x + 2 ] 0 n = lim n [ ln x 2 + 5 ( 3 x + 2 ) 2 ] 0 n = lim n [ ln ( n 2 + 5 9 n 2 + 12 n + 4 ) ln ( 5 4 ) ] = ln 1 9 ln 5 4 = ln 4 45 = 2.42 (3.s.f) \begin{aligned}\int_0^\infty\frac{4x-30}{(x^2+5)(3x+2)}dx&=\int_0^\infty\frac{2x}{x^2+5}-\frac{6}{3x+2}dx\\&=\lim_{n\to\infty}\int_0^n\frac{2x}{x^2+5}-\frac{6}{3x+2}dx\\&=\lim_{n\to\infty}\left [ \ln|x^2+5|-2\ln|3x+2| \right ]_0^n\\&=\lim_{n\to\infty}\left [\ln\frac{x^2+5}{(3x+2)^2}\right ]_0^n\\&=\lim_{n\to\infty}\left[\ln\left (\frac{n^2+5}{9n^2+12n+4}\right )-\ln\left (\frac{5}{4}\right )\right ]\\&=\ln\frac{1}{9}-\ln\frac{5}{4}\\&=\ln\frac{4}{45}=\color{#20A900}\boxed{-2.42\ \textnormal{(3.s.f)}}\end{aligned}

Gah, sorry - we typed identical solutions at identical times! You've got more detail in yours so I'll delete mine. Nice problem, by the way - the need to check behaviour in the limit is a nice aspect.

Chris Lewis - 9 months ago

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