You will be trapped with this Integral

Calculus Level 3

Find the value of this integral 0 1 6 x 2 x 1 6 x 2 5 x + 1 d x \displaystyle \int_0^1 \frac{6x^2-x-1}{6x^2-5x+1} \, dx

0 1 + 2 3 ln 2 \displaystyle 1+\frac{2}{3}\ln 2 1 -\infty divergent + +\infty

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1 solution

The integrand can be rewritten as

6 x 2 5 x + 1 + 4 x 2 6 x 2 5 x + 1 = 1 + 2 ( 2 x 1 ) ( 2 x 1 ) ( 3 x 1 ) = 1 + 2 3 x 1 \dfrac{6x^{2} - 5x + 1 + 4x - 2}{6x^{2} - 5x + 1} = 1 + \dfrac{2(2x - 1)}{(2x - 1)(3x - 1)} = 1 + \dfrac{2}{3x - 1}

for x 1 2 x \ne \frac{1}{2} . The point of discontinuity ( 1 2 , 5 ) (\frac{1}{2}, 5) does not affect the integral, but what is of concern is when 3 x 1 = 0 3x - 1 = 0 , as this leads to the vertical asymptote x = 1 3 x = \frac{1}{3} . So splitting the integral into two improper integrals, one from 0 0 to 1 3 \frac{1}{3} and the other from 1 3 \frac{1}{3} to 1 1 , we end up needing to evaluate

( x + 2 3 ln 3 x 1 ) 0 1 3 + ( x + 2 3 ln 3 x 1 ) 1 3 + 1 = (x + \frac{2}{3}\ln |3x - 1|)_{0}^{\frac{1}{3}^{-}} + (x + \frac{2}{3}\ln |3x - 1|)_{\frac{1}{3}^{+}}^{1} =

1 3 + 2 3 lim x 1 3 ln ( 1 3 x ) + 1 + 2 3 ln ( 2 ) 1 3 2 3 lim x 1 3 + ln ( 3 x 1 ) = \displaystyle \dfrac{1}{3} + \dfrac{2}{3}\lim_{x \to \frac{1}{3}^{-}} \ln(1 - 3x) + 1 + \dfrac{2}{3}\ln(2) - \dfrac{1}{3} - \dfrac{2}{3}\lim_{x \to \frac{1}{3}^{+}} \ln(3x - 1) =

1 + 2 3 ln ( 2 ) + 2 3 ( lim x 1 3 ln ( 1 3 x ) lim x 1 3 + ln ( 3 x 1 ) ) \displaystyle 1 + \dfrac{2}{3}\ln(2) + \dfrac{2}{3}\left(\lim_{x \to \frac{1}{3}^{-}} \ln(1 - 3x) - \lim_{x \to \frac{1}{3}^{+}} \ln(3x - 1)\right) .

Now the bracketed pair of limits are of the indeterminate form ( ) -\infty - (-\infty) \Longrightarrow \infty - \infty , and so the integral cannot be evaluated, i.e., it is d i v e r g e n t \boxed{divergent} .

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