Integral I

Calculus Level 2

0 2 x + 1 x 2 + x + 2 dx \int_0^2 \frac{x+1}{x^2+x+2} \text{dx}

Find the value of the integral above to three significant digits.


The answer is 0.967.

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1 solution

I = 0 2 x + 1 x 2 + x + 2 d x = 1 2 0 2 ( 2 x + 1 x 2 + x + 2 + 1 x 2 + x + 2 ) d x = ln ( x 2 + x + 2 ) 2 0 2 + 1 2 0 2 1 ( x + 1 2 ) 2 + 7 4 d x = ln 2 + 2 7 0 2 1 ( 2 x + 1 7 ) 2 + 1 d x Let tan θ = 2 x + 1 3 = ln 2 + 1 7 tan 1 1 7 tan 1 5 7 d θ sec 2 θ d θ = 2 3 d x = ln 2 + 1 7 ( tan 1 5 7 tan 1 1 7 ) 0.966 \begin{aligned} I & = \int_0^2 \frac {x+1}{x^2 + x + 2} dx \\ & = \frac 12 \int_0^2 \left(\frac {2x + 1}{x^2+x+2} + \frac 1{x^2+x+2} \right) dx \\ & = \frac {\ln(x^2+x+2)}2 \bigg|_0^2 + \frac 12 \int_0^2 \frac 1{\left(x+\frac 12\right)^2 + \frac 74} dx \\ & = \ln 2 + \color{#3D99F6} \frac 27 \int_0^2 \frac 1{\left(\frac {2x+1}{\sqrt 7}\right)^2 + 1} dx & \small \color{#3D99F6} \text{Let }\tan \theta = \frac {2x+1}{\sqrt 3} \\ & = \ln 2 + \color{#3D99F6} \frac 1{\sqrt 7} \int_{\tan^{-1} \frac 1{\sqrt 7}}^{\tan^{-1} \frac 5{\sqrt 7}} d\theta & \small \color{#3D99F6} \implies \sec^2 \theta \ d\theta = \frac 2{\sqrt 3} dx \\ & = \ln 2 + \frac 1{\sqrt 7} \left(\tan^{-1} \frac 5{\sqrt 7} - \tan^{-1} \frac 1{\sqrt 7} \right) \\ & \approx \boxed{0.966} \end{aligned}

@Chew-Seong Cheong Minor typo in the last line. It should read tan 1 5 7 tan 1 1 7 \displaystyle \tan^{-1}\frac{5}{\sqrt{7}}-\tan^{-1}\frac{1}{\sqrt{7}}

Aaghaz Mahajan - 1 year, 11 months ago

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Thanks. I have changed it.

Chew-Seong Cheong - 1 year, 11 months ago

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