# GMU Math MakerLab

## Friday, January 27, 2017

## Tuesday, November 22, 2016

### Volume by Disc and Shell Methods

**Volume by Disc and Shell Methods**

**6.3**Volume by slicing

**6.4**Volume by shells

Note: Section numbers refers to Calculus, Early Transcendentals (2nd edition) by Briggs, Cochran, and Gillett.

These prints illustrate the disc method and the shell method for finding volumes of solids of revolution.

The blue object is obtained by revolving the region between y-axis and the graph of:

The model on the left shows the solid. The one on the right is an approximation using 10 shells.

In the solid below (titled the Volumes of Hanoi) the piece on the left of the model is a solid of revolution obtained by rotating the region bounded by the graph of the function

The rightmost set of pieces represents an approximation of volume of the center solid of revolution with four discs, and the middle set represents a different approximation of that solid with four shells. The discs each have the same height but their radii are determined by the function f(x). The shells each have the same thickness but their heights are determined by the function f(x).

On one side is the approximation of the volume using four disks, and on the other side is the approximation of the volume using four shells. To make it clear which is which, we have shown the same model with the pieces taken apart.

The next model is a solid obtained by revolving the region between the graph of

The model on the left is the exact region, and the one on the right is an approximation which uses 8 shells.

### Volume by slicing

**Volume by slicing**

**Sec 6.3**Volume by slicing, Q10, pg 430

For this solid, it is possible to find the volume by integration of the cross sectional area. The solid has a circular base of radius r. The cross sections perpendicular to the base and parallel to the z-axis are equilateral triangles.

Four slice version by Evelyn Sander.

### Double Integrals over Rectangular Regions

Double Integrals over Rectangular Regions

Double Integrals over Rectangular Regions

**Section 13.1**: Double Integrals over Rectangular Regions

Note: Section number refers to Calculus, Early Transcedentals (2nd edition) by Briggs, Cochran, and Gillett.

These 3D prints illustrate the concept of

**Riemann sums**used for calculating volumes of solids. As the number of rectangles are increased, we get a better and better approximation of the double integral.
The double integral approximation for the function:

is demonstrated below. It is the Gaussian curve in 3 dimensions, generated using 100, 225, and 400 rectangular prisms, respectively on the interval [-2,2.5] x [-2,2]. The Mathematica code used to generate these models was based on Raouf Boules', Geoff Goodson's, Ohoe Kim's and Mike O'Leary's Calculus III Lab at Towson University, found here.

The integral approximator (code provided below) takes a function f on an interval [a1,b1]x[a2,b2] with n discretization points in both the x and y directions, therefore approximating the area under the function with n^2 rectangular prisms (cuboids in Mathematica). Just like in the 1-dimensional integral approximation methods,

The x and y coordinates of the jth and kth rectangle are thus given by,

**Technical Details for Printing:**

In Mathematica, Cuboid[{lower corner (x,y,z), upper corner (x,y,z)]. Here the lower corner is given by (x_j, y_k, 0), and the upper corner (x(j+1), y_(k+1), f(x_j, y_k)).

**Mathematica code:**

RD[f

*, {a1*, b1*}, {a2*, b2*}, n*] := Show[Table[Graphics3D[Cuboid[{a1 + (b1 - a1)/n*j, a2 + (b2 - a2)/n*k, 0},
{a1 + (b1 - a1)/n

*(j + 1), a2 + (b2 - a2)/n*(k + 1),f[a1 + (b1 - a1)/n*j, a2 + (b2 - a2)/n*k]}]], {j, 0, n - 1}, {k,0, n - 1}]]
Clear[f, x, y]

f[x

*, y*] := 2*Exp[-x^2 - y^2]
print1 = RD[f, {-2, 2.5}, {-2, 2}, 10]

Export["filename.stl",print1]

In the figure below, from left to right, we have n=10, 15, 20 and 50.

### Volume by Cross Section

**Volume by Cross Section**

**6.3**Volume by slicing

Note: Section numbers refers to Calculus, Early Transcendentals (2nd edition) by Briggs, Cochran, and Gillett.

These prints illustrate a solid whose base is the region bounded by

The cross sections perpendicular to the base and parallel to the x-axis are squares.

While it is possible to find the volume of this shape by via a cross section integral formula without ever visualizing it, it is a more satisfying experience if it is clear what it looks like.

### Model credits: UNCC Calculus 2, Janthefischer, January 11, 2016

.### Saddle

**Saddles**

**12.1**Planes and surfaces

**12.2**Graphs and level curves

**12.6**Directional derivatives and the gradient

**12.8**Maximum/minimum problems

Note: Section numbers refers to Calculus, Early Transcendentals (2nd edition) by Briggs, Cochran, and Gillett.

**Saddle surface**

Saddle with grid lines and saddle with level sets |

**quartic surface**: a standard mathematical saddle surface. This surface is the graph of the function

In the red and blue version, the red curves second color depicts the level sets of the surface. In the blue and white version, the blue curves depict the fixed x and fixed y grid lines. This point (x,y,z) = (0,0,1) (the middle point) is a critical point, meaning all partial derivatives are zero, but the point is neither a maximum nor a minimum.

**Monkey Saddle**

The following print illustrates the graph of the function

This is a

**cubic surface**. Like the standard saddle, it still has a critical point at (x,y) = (0,0), (the center), but it is even flatter than the standard saddle. It is called a monkey saddle since a monkey could sit on this saddle with room for both legs and its tail.

**Technical Details for Printing**

**These objects were designed in Mathematica. These technical notes were determined by trial and error (or more accurately error and error and error ad nauseam)**

- The parametrization lines are created using the Tube command in Mathematica.
- This was printed with neither raft nor supports.
- The blue base piece is needed in order to make the print work without falling over, the thickness of the white surface is 2, and the thickness of the blue parametrization is 2.5.
- In order to make the blue base pieces, I made spheres in Mathematica and used the hole options in Tinkercad to cut off the bottom half. Do not just leave the sphere intact and assume that the printer will ignore the part below the build plate. This actually seems to stop the printer for a long time and set there thinking about printing the bottom half of the sphere.
- Do not try to print something too steep - it seems that if it's about the same height as length and width you'll be better off. A steep piece can break off the base under its own weight. On the other hand, too wide will give the dreaded overhangs of more than 45 degrees.
- The side walls that are printed on the dual print can topple over. This seems to be a build plate adhesion problem. No consistent solution to this problem.

**Quartic Surface with Level Sets**

**Technical Details for Printing the monkey saddle**

The standard monkey saddle function does not include the 1/2, but this made for too steep a figure, which started to break off at the corners.

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