MHB Jake's questions about integrations

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The discussion focuses on calculating volumes of solids formed by rotating functions around specified axes. The first volume is derived from the function y = 4 - x², yielding a result of approximately 70.37 cubic units after integrating over the interval [0, 2]. The second volume involves the function y = 3 + (1/4)√x, with integration performed over the interval [1, 3/2], resulting in approximately 12.88 cubic units. Key techniques include using cylindrical shells and applying the disk method for volume calculations. The calculations emphasize the importance of correctly setting up the integrals based on the given functions and intervals.
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5. To start with, we should work out the x intercepts, they are x = -2 and x = 2. That means your region in the first quadrant will be integrated over $\displaystyle \begin{align*} x \in [0,2] \end{align*}$.

You should note that rotating the function $\displaystyle \begin{align*} y = 4 - x^2 \end{align*}$ about the line $\displaystyle \begin{align*} y = -\frac{1}{2} \end{align*}$ will give the exact same volume as rotating $\displaystyle \begin{align*} y = \frac{9}{2} - x^2 \end{align*}$ around the x axis. We would then subtract the volume of the region bounded by the line $\displaystyle \begin{align*} y = \frac{1}{2} \end{align*}$ rotated about the x axis.

So our required volume is

$\displaystyle \begin{align*} V &= \int_0^2{ \pi\,\left( \frac{9}{2} - x^2 \right) ^2 \,\mathrm{d}x } - \int_0^2{ \pi\,\left( \frac{1}{2} \right) ^2\,\mathrm{d}x } \\ &= \pi \int_0^2{ \left[ \left( \frac{9}{2} - x^2 \right) ^2 - \left( \frac{1}{2} \right) ^2 \right] \,\mathrm{d}x } \\ &= \pi \int_0^2{ \left( \frac{81}{4} - 9\,x^2 + x^4 - \frac{1}{4} \right) \,\mathrm{d}x } \\ &= \pi \int_0^2{ \left( 20 - 9\,x^2 + x^4 \right) \,\mathrm{d}x } \\ &= \pi \,\left[ 20\,x - 3\,x^3 + \frac{x^5}{5} \right] _0^2 \\ &= \pi \,\left[ \left( 20 \cdot 2 - 3 \cdot 2^3 + \frac{2^5}{5} \right) - \left( 20 \cdot 0 - 3 \cdot 0^3 + \frac{0^5}{5} \right) \right] \\ &= \pi \, \left( 40 - 24 + \frac{32}{5} - 0 \right) \\ &= \pi \, \left( 16 + \frac{32}{5} \right) \\ &= \frac{112\,\pi}{5}\,\textrm{units}^3 \\ &\approx 70.371\,68 \, \textrm{units}^3 \end{align*}$I will do the second question when I have a spare moment.
 

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To evaluate the second volume, you need to imagine the region being made up of a very large number of vertically oriented cylinders. The areas of the curved surfaces of the cylinders together build up to the volume of your solid.

In each cylinder, the radius is the x value, and the height is the y value. So each cylinder has area $\displaystyle \begin{align*} 2\,\pi\,x\,y \end{align*}$, where $\displaystyle \begin{align*} y = 3 + \frac{1}{4}\,\sqrt{x} \end{align*}$ and $\displaystyle \begin{align*} x \in \left[ 1, \frac{3}{2} \right] \end{align*}$. Thus the volume is

$\displaystyle \begin{align*} V &= \int_1^{\frac{3}{2}}{ 2\,\pi\,x\,\left( 3 + \frac{1}{4}\,\sqrt{x} \right) \,\mathrm{d}x } \\ &= 2\,\pi\int_1^{\frac{3}{2}}{ \left( 3\,x + \frac{1}{4}\,x^{\frac{3}{2}} \right) \,\mathrm{d}x } \\ &= 2\,\pi\,\left[ \frac{3\,x^2}{2} + \frac{1}{4}\,\left( \frac{x^{\frac{5}{2}}}{\frac{5}{2}} \right) \right]_1^{\frac{3}{2}} \\ &= 2\,\pi\,\left[ \frac{3\,x^2}{2} + \frac{x^{\frac{5}{2}}}{10} \right] _1^{\frac{3}{2}} \\ &= 2\,\pi\,\left\{ \left[ \frac{3\,\left( \frac{3}{2} \right) ^2}{2} + \frac{\left( \frac{3}{2} \right) ^{\frac{5}{2}}}{10} \right] - \left[ \frac{3\,\left( 1 \right) ^2}{2} + \frac{1^{\frac{5}{2}}}{10} \right] \right\} \\ &= 2\,\pi \,\left( \frac{27}{8} + \frac{9\,\sqrt{6}}{80} - \frac{3}{2} - \frac{1}{10} \right) \\ &= \pi\,\left( \frac{27}{4} + \frac{9\,\sqrt{6}}{40} - 3 - \frac{1}{5} \right) \\ &= \pi \,\left( \frac{270}{40} + \frac{9\,\sqrt{6}}{40} - \frac{120}{40} - \frac{8}{40} \right) \\ &= \frac{\left( 142 + 9\,\sqrt{6} \right)\,\pi }{40}\,\textrm{units}^3 \\ &\approx 12.884\,10\,\textrm{units}^3 \end{align*}$
 
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