Solid volume using double integral I have to calculate the volume of the solid bounded by
$$x^{2}+y^{2}+z^{2}=6 \qquad z=x^{2}+y^{2} \qquad z\geq0$$
using double integrals.
When I drew it, I could see that a part of the sphere is above the paraboloid. So, for me, it should be something like that:
$$V=\iint_{B}\left[\sqrt{6-x^{2}-y^{2}}-\left(x^{2}+y^{2}\right)\right]dx\,dy$$
But my problem is to find these integrals. I think I'm supposed to not use spheric coordinates (since I'm calculating a double integral, not a triple one), but even the polar ones, I don't know how to find them.
 A: Polar coordinates
$x = r\cos \theta\\
y = r\sin \theta\\
dx\ dy = r \ dr\ dz$
$\iint (\sqrt {6-r^2} - r^2) r\ dr\ d\theta$
Limits:
Find where the two curves intersect.
Substitute $x^2+ y^2 = z$ into the equation of the sphere.
$z + z^2 = 6\\
(z + 3)(z-2) = 0\\
z = 2\\
r^2 = 2$
$\int_0^{2\pi}\int_0^{\sqrt 2} r\sqrt {6-r^2} - r^3 \ dr\ d\theta$
A: $\newcommand{\bbx}[1]{\,\bbox[15px,border:1px groove navy]{\displaystyle{#1}}\,}
 \newcommand{\braces}[1]{\left\lbrace\,{#1}\,\right\rbrace}
 \newcommand{\bracks}[1]{\left\lbrack\,{#1}\,\right\rbrack}
 \newcommand{\dd}{\mathrm{d}}
 \newcommand{\ds}[1]{\displaystyle{#1}}
 \newcommand{\expo}[1]{\,\mathrm{e}^{#1}\,}
 \newcommand{\ic}{\mathrm{i}}
 \newcommand{\mc}[1]{\mathcal{#1}}
 \newcommand{\mrm}[1]{\mathrm{#1}}
 \newcommand{\pars}[1]{\left(\,{#1}\,\right)}
 \newcommand{\partiald}[3][]{\frac{\partial^{#1} #2}{\partial #3^{#1}}}
 \newcommand{\root}[2][]{\,\sqrt[#1]{\,{#2}\,}\,}
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Given the problem symmetry, it's convenient to use cylindrical coordinates
  $\ds{\pars{\rho, \phi, z}}$. Hereafter, $\ds{\bracks{\cdots}}$ is an
  Iverson Bracket which is a $\textsf{quite convenient 'tool' to handle constraints}$.

\begin{align}
V & =
\iiint_{\mathbb{R}^{3}}\bracks{x^{2} + y^{2}  + z^{2} < 6}
\bracks{z > x^{2} + y^{2}}\bracks{z \geq 0}\dd x\,\dd y\,\dd z
\\[5mm] & =
\int_{0}^{\infty}\int_{0}^{2\pi}\int_{0}^{\infty}
\bracks{\rho^{2}+ z^{2} < 6}
\bracks{z > \rho^{2}}\rho\,\dd\rho\,\dd\phi\,\dd z
\end{align}
Since the integrand is $\ds{\phi}$-independent
$\ds{\pars{~\mbox{In addittion, I'll make the change}\ \rho^{2}\ \mapsto\ \rho~}}$:
\begin{align}
V & =
\pi\int_{0}^{\infty}\int_{0}^{\infty}
\bracks{z < \root{6}}\bracks{\rho < 6 - z^{2}}\bracks{\rho < z}
\,\dd\rho\,\dd z
\\[5mm] & =
\pi\int_{0}^{\root{6}}\int_{0}^{\infty}
\bracks{\rho < 6 - z^{2}}\bracks{\rho < z}\,\dd\rho\,\dd z
\\[5mm] & =
\pi\int_{0}^{\root{6}}\
\overbrace{\bracks{6 - z^{2} < z}}
^{\ds{\pars{z < - 3}\ \mbox{or}\ \pars{z > 2}}}\
\int_{0}^{6 - z^{2}}\,\dd\rho\,\dd z\ +\
\pi\int_{0}^{\root{6}}\
\overbrace{\bracks{6 - z^{2} > z}}^{\ds{-3 < z < 2}}\
\int_{0}^{z}\,\dd\rho\,\dd z
\\[5mm] & =
\pi\, \underbrace{\int_{2}^{\root{6}}\pars{6 - z^{2}}\dd z}
_{\ds{4\root{6} - {28 \over 3}}}\ +\
\pi\ \underbrace{\int_{0}^{2}z\,\dd z}_{\ds{2}} =
\bbx{4\root{6}\pi - {22 \over 3}\,\pi} \approx 7.7429
\end{align}
A: In the case that the solid in question is bounded between the graphs of two functions, say $f$ and $g$, calculating the volume with double integrals (integrating $f-g$) is the same as using triple integrals (integrating the function $1$ on an appropriate set).
Anyhow, to use polar coordinates, you need to to find the maximum possible value of the radius, since it is geometrically clear that
$$0<\theta<2\pi$$
$$0<r<R$$
for some $R$. To find this $R$, substitute the second equation in the first. This should give you a circle in $x$ and $y$, which is to be expected (draw a picture if this is not clear). The radius of said circle is your $R$.
A: So your integral you write down is correct. And it is easy to asses that the radius of the circle you have to integrate over is $R=\sqrt{2}$ since :
$$ x^2+y^2 = \sqrt{6-x^2-y^2},$$
$$ r^2 = \sqrt{6-r^2} $$
Thus your integral is now directly solvable :
$$\iint_\mathcal{B} \sqrt{6-x^2-y^2} - x^2-y^2 \textrm{d}x\textrm{d}y,$$
$$\int_0^{2\pi}\int_0^R \left(\sqrt{6-r^2} - r^2\right)r\, \textrm{d}r\textrm{d}\theta,$$
$$2\pi\left(2\sqrt{6}-\frac{\sqrt{6-R^2}^3}{3}-\frac{R^4}{4}\right)$$
Which leads to the final result :
$$I=\pi\left(4\sqrt 6-\frac{22}3\right)$$
In the second step we switched to polar coordinates using:
$$\iint f(x,y)\,\textrm{d}x\textrm{d}y = \iint f(x(r,\theta),y(r,\theta)) r\,\textrm{d}r\textrm{d}\theta .$$
