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THIS QUESTION IS KILLING ME
Calculate the volume of the object by using the triple integral.

THIS QUESTION IS KILLING ME Calculate the volume of the object by using the triple integral class=

Respuesta :

The volume of the solid (call it S) in Cartesian coordinates is

[tex]\displaystyle\iiint_S\mathrm dV=\int_{-1}^1\int_{-\sqrt{1-x^2}}^{\sqrt{1-x^2}}\int_{(x^2+y^2)^2-1}^{4-4(x^2+y^2)}\mathrm dz\,\mathrm dy\,\mathrm dx[/tex]

but I suspect converting to cylindrical coordinates would make the integral much more tractable.

Take

[tex]\begin{cases}x=r\cos\theta\\y=r\sin\theta\\z=z\end{cases}\implies\mathrm dV=r\,\mathrm dr\,\mathrm d\theta\,\mathrm dz[/tex]

Then

[tex]4-4(x^2+y^2)=4-4r^2=4(1-r^2)[/tex]

[tex](x^2+y^2)^2-1=(r^2)^2-1=r^4-1[/tex]

and the integral becomes

[tex]\displaystyle\iiint_S\mathrm dV=\int_0^{2\pi}\int_0^1\int_{r^4-1}^{4(1-r^2)}r\,\mathrm dz\,\mathrm dr\,\mathrm d\theta[/tex]

[tex]=\displaystyle2\pi\int_0^1r(4(1-r^2)-(r^4-1))\,\mathrm dr[/tex]

[tex]=\displaystyle2\pi\int_0^1r(5-4r^2-r^4)\,\mathrm dr[/tex]

[tex]=\displaystyle2\pi\int_0^15r-4r^3-r^5\,\mathrm dr[/tex]

[tex]=2\pi\left(\dfrac52-1-\dfrac16\right)=\boxed{\dfrac{8\pi}3}[/tex]