Calculate the volume bounded by $z = 0$, $z = y^2$, $x^2+y^2=1$

calculusintegrationvolume

Calculate the volume of the solid bounded by these surfaces:

$z = 0$,
$z = y^2$,
$x^2+y^2=1$

I visualized the surfaces and came to a conclusion of this integral:

$\iint{dxdy}\int_{0}^{y^2}dz$

After calculating it and swapping to $x=r \cos {\theta}$, $x=r \sin {\theta}$, I finished with the result of $\frac{\pi}{3}$. Is this the correct way of calculating the result of this exercise and did I arrive at the correct conclusion at the end?

Thank you.

Best Answer

If you express your region in cylindrical coordinates, then $x^2+y^2=1$ becomes $\rho=1$ and $z=y^2$ becomes $z=\rho^2\sin^2(\theta)$. So, you can compute your volume as follows:\begin{align}\int_0^{2\pi}\int_0^1\int_0^{\rho^2\sin^2(\theta)}\rho\,\mathrm dz\,\mathrm d\rho\,\mathrm d\theta&=\int_0^{2\pi}\int_0^1\rho^3\sin^2(\theta)\,\mathrm d\rho\,\mathrm d\theta\\&=\left(\int_0^{2\pi}\sin^2(\theta)\,\mathrm d\theta\right)\left(\int_0^1\rho^3\,\mathrm d\rho\right)\\&=\frac\pi4.\end{align}

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