Indefinite integral

Calculus Level 5

Let f ( x ) = x 7 + 2 ( x 2 + x + 1 ) 2 d x \displaystyle f(x) = \int \frac {x^7+2}{(x^2+x+1)^2} dx , subject to f ( 0 ) = π 3 3 f(0) = \dfrac{ \pi } { 3 \sqrt{3} } .

Find f ( 1 ) |f(-1) | .


The answer is 2.187.

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1 solution

x 7 + 2 ( x 2 + x + 1 ) 2 d x 2 log ( x 2 + x + 1 ) + 1 12 x ( 3 x 3 8 x 2 + 12 x 2 + x + 1 + 6 x + 24 ) + 2 tan 1 ( 2 x + 1 3 ) 3 \int \frac{x^7+2}{\left(x^2+x+1\right)^2} \, dx \Longrightarrow -2 \log \left(x^2+x+1\right)+\frac{1}{12} x \left(3 x^3-8 x^2+\frac{12}{x^2+x+1}+6 x+24\right)+\frac{2 \tan ^{-1}\left(\frac{2 x+1}{\sqrt{3}}\right)}{\sqrt{3}}

As that expression at x = 0 π 3 3 x=0 \Longrightarrow \frac{\pi }{3 \sqrt{3}} , no further correction is needed as an integration constant.

19 12 π 3 3 2.18793312141 -\frac{19}{12}-\frac{\pi }{3 \sqrt{3}} \approx -2.18793312141 . The absolute value was requested as the answer.

x 7 + 2 ( x 2 + x + 1 ) 2 = = x 3 2 x 2 + x + 2 ( x 2 + x + 1 ) 2 2 ( 2 x + 1 ) x 2 + x + 1 + x + 2 \frac{x^7+2}{\left(x^2+x+1\right)^2} == x^3-2 x^2+\frac{x+2}{\left(x^2+x+1\right)^2}-\frac{2 (2 x+1)}{x^2+x+1}+x+2 2 x + 1 x 2 + x + 1 d x , u = x 2 + x + 1 1 u d u ln u \int \frac{2 x+1}{x^2+x+1} \, dx, u=x^2+x+1 \Longrightarrow \int \frac{1}{u} \, du \longrightarrow \Longrightarrow \ln{u} x + 2 ( x 2 + x + 1 ) 2 2 x + 1 2 ( x 2 + x + 1 ) 2 + 3 2 ( x 2 + x + 1 ) 2 \frac{x+2}{\left(x^2+x+1\right)^2} \Longrightarrow \frac{2x+1}{2\left(x^2+x+1\right)^2}+\frac{3}{2\left(x^2+x+1\right)^2} 2 x + 1 ( x 2 + x + 1 ) 2 d x , s = x 2 + x + 1 1 s 2 d s \int \frac{2 x+1}{\left(x^2+x+1\right)^2} \, dx, s=x^2+x+1 \Longrightarrow \int \frac{1}{s^2} \, ds 1 ( ( x + 1 2 ) 2 + 3 4 ) 2 d x , p = x + 1 2 , p = 1 2 3 ( tan w ) , cos 2 ( w ) = 1 2 ( 2 w ) cos + 1 2 16 9 ( w cos 2 ) d w \int \frac{1}{\left(\left(x+\frac{1}{2}\right)^2+\frac{3}{4}\right)^2} \, dx, p=x+\frac12, p=\frac{1}{2} \sqrt{3} (\tan w), \cos ^2(w)=\frac{1}{2} (2 w) \cos +\frac{1}{2} \Longrightarrow \int \frac{16}{9} \left(w \cos ^2\right) \, dw

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