# Math Help - Voulme by integration

1. ## Voulme by integration

A glass vase has the shape of the solid obtained by rotating about the y-axis the area in the first quadrant lying over the x-interval [0,a] and under the graph of $y=\sqrt{x}$

Determine how much glass is contained in the vase.

2. Originally Posted by Exotique
A glass vase has the shape of the solid obtained by rotating about the y-axis the area in the first quadrant lying over the x-interval [0,a] and under the graph of $y=\sqrt{x}$

Determine how much glass is contained in the vase.
Hi Exotique,

$y=\sqrt{x}\ \Rightarrow\ x=y^2$

As the curve is being rotated about the y-axis, we need the limits on the y-axis.
Also, we integrate the discs with radius=x.

$x=0,\ y=0$

$x=a,\ y=\sqrt{a}$

The volume of the vase is $\int_{y=0}^{\sqrt{a}}{\pi}x^2dy$

$={\pi}\int_{0}^{\sqrt{a}}y^4dy$

If the glass is the solid part under the graph above the x-axis, with a hollow interior above the graph,

subtract this integral from ${\pi}a^2(\sqrt{a})$

3. The value I obtained was
$\pi\sqrt{a}^5-\frac{1}{5}\pi\sqrt{a}^5$
$=\frac{4\pi}{5}\cdot\sqrt{a}^5$

Is this correct?

4. Originally Posted by Exotique
The value I obtained was
$\pi\sqrt{a}^5-\frac{1}{5}\pi\sqrt{a}^5$
$=\frac{4\pi}{5}\cdot\sqrt{a}^5$

Is this correct?
Yes,

if the vase really is that shape.

We get the same answer by integrating the surface areas of the washers
(denoted by the 2 red discs).

This is equivalent to subtracting the interior volume (of air)
from the volume of a cylinder.

$\int_{0}^{\sqrt{a}}\left({\pi}a^2-{\pi}x^2\right)dy={\pi}\int_{0}^{\sqrt{a}}\left(a^ 2-x^2\right)dy$

$={\pi}\int_{0}^{\sqrt{a}}\left(a^2-y^4\right)dy={\pi}(\sqrt{a})^5-{\pi}\frac{(\sqrt{a})^5}{5}$