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Math Help - 2 easy double integrals

  1. #1
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    2 easy double integrals

    Hello and thanks in advance for your help !

    I need those 2 integrals solved until tonight , cause tomorrow morning i'm writing a text and i need them.have tried and failed so far !

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  2. #2
    Senior Member DeMath's Avatar
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    Re: 2 easy double integrals

    Hint for the second integral: change the order of integration

    \begin{gathered}I= \int\limits_0^1 {\int\limits_0^3 {x\sqrt {{x^2} + {y^2}}\,dydx} }  = \int\limits_0^3 {\int\limits_0^1 {x\sqrt {{x^2} + {y^2}}\,dxdy} }  \hfill \\[5pt] \left[ \begin{gathered} \sqrt {{x^2} + {y^2}}  = t~~ \Rightarrow~~ {x^2} + {y^2} = {t^2}~~ \Rightarrow ~~x\,dx = t\,dt \hfill \\\int {x\sqrt {{x^2} + {y^2}} dx}  = \int {{t^2}dt}  = \frac{{{t^3}}}{3} + C = \frac{1}{3}{({x^2} + {y^2})^{3/2}} + C \hfill \\ \end{gathered}  \right] \hfill \\[5pt] I= \int\limits_0^3 {dy} \left. {\frac{1}{3}{{({x^2} + {y^2})}^{3/2}}} \right|_{x = 0}^{x = 1} = \frac{1}{3}\int\limits_0^3 {\left[ {{{(1 + {y^2})}^{3/2}} - {y^3}} \right]dy=\ldots=\frac{23}{8}\sqrt{10}-\frac{27}{4}-\frac{1}{8}\ln(\sqrt{10}-3)}  \hfill \\ \end{gathered}
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  3. #3
    Senior Member DeMath's Avatar
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    Re: 2 easy double integrals

    The first integral

    \begin{aligned}\int\limits_1^3 \int\limits_0^x \frac{2\,dydx}{\sqrt{x^2+y^2}}&= 2\int\limits_1^3 dx\! \left. {\ln\Bigl(y+\sqrt{x^2+y^2}}\Bigr)} \right|_{y = 0}^{y = x} = 2\int\limits_1^3 \Bigl[\ln (x + \sqrt 2 x) - \ln x\Bigr]dx=\\[2pt] &= 2\int\limits_1^3 \Bigl[\ln x + \ln (1 + \sqrt 2 ) - \ln x\Bigr]dx=  4\ln (1 + \sqrt 2 ) \end{aligned}
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    Re: 2 easy double integrals

    thank you so much :-))))) ! just solved myself the second one but really thanks for both ! didn't expect such a quick answer !
    but concerning first integral my book gives us answer (pi/2)*ln3 ...
    Last edited by coutmath; May 7th 2012 at 03:23 AM.
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  5. #5
    Senior Member DeMath's Avatar
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    Re: 2 easy double integrals

    but concerning first integral my book gives us answer (pi/2)*ln3 ...
    I checked in Maple. Check carefully.
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