The Math Forum

Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Math Forum » Discussions » Education » math-teach

Topic: Lesson Plan (alternative models of multiplying to get area)
Replies: 16   Last Post: Aug 22, 2017 11:20 PM

Advanced Search

Back to Topic List Back to Topic List Jump to Tree View Jump to Tree View   Messages: [ Previous | Next ]
kirby urner

Posts: 3,205
Registered: 11/29/05
Lesson Plan (alternative models of multiplying to get area)
Posted: Aug 17, 2017 12:53 PM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply
att1.html (4.9 K)

Lesson: Alternative Geometric Models of Multiplication

In this Lesson, we articulate a different set of assumptions and show that
internal consistency is maintained.

A goal of this lesson is to show that altering assumptions and definitions
may lead to inconsistencies, or to whole new geometries or other
mathematical systems.[1]

[ Animation (optional) ]

Consider a tribe, cult or culture that uses triangles a lot, especially
right triangles, and in one of their language games, the right triangle
with edges 3, 4 is said to have area 12.

Simply multiply the two edges stemming from the single right angle. Do not
divide the result by 2 as we normally would.

Numeric results have not changed, relative to our own culture's, with its
rectilinear conventions, only the geometric representations of products. 3
x 4 is still 12.

[ Pause for analysis and discussion ]

In this game, what would be an expression for the area of a rectangle with
edges AB.


Now consider a tribe, cult or culture that uses a 60 degree angle and
defines the unit of area to be an equilateral triangle of edges one.

Like the tribe above, a triangle defined by two segments at 60 degrees,
edges 3 and 4, is said to have area 12. [2]

[ Pause for analysis and discussion ]

What conversion constant would take us from area AB in the sixty degree
system, to area AB in the ninety degree system.



What conversion constant would take us from a rectilinear depiction of area
AB, such as our culture uses (e.g. "two by four" means a rectangular
cross-section of a beam), to the corresponding area in the sixty degree


End of Part One.

Teacher notes:

in both cases, you may want to define triangular multiplication in terms of
"adding a lid" or "closing off" or "building a third fence to create an

Allow that these tribes may be agricultural and tend to omni-triangulate
with their fences and/or land-survey boundaries.

Connect to the XYZ coordinate system and have students consider volume ABC
in terms of (a) a right tetrahedron of edges A,B,C from the origin
(b) a tetrahedron sliced from a more gigantic regular one i.e. angle AB,
AC, BC are all sixty degrees.

We are building a foundation for a future Lesson (Part Two) in which the
regular tetrahedron plays the role of unit volume. That's our entry point
into the concentric hierarchy (focal point).

Note that our conversion constants depend on different notions of "unit"
and we may find a convenient way of relating linear as well as areal or
volumetric units.

A sphere radius might count as unit in one system, while the diameter
counts as one in another. See: pi versus tau controversy. (Vi Hart)


Follow-up Resources:

[2] Tropical Math (animation, Khan academy style)

Homework reading:

Possible next Lessons:

* Omni-triangulating a sphere: different strategies -- Lat / long is about
dividing the Earth's surface into spherical trapezoids...

* Topology of Nodes, Edges: Bridges of Königsberg ; NYC subways

* Position vectors in XYZ: basis vectors OA, OB, OC are unit length;
space-filling cubes; negative basis vectors not considered basis vectors

* Quadray coordinates in the IVM: the four basis vectors need not be unit
vis-a-vis XYZ, with containing tetrahedron edges = D (CCP ball diameter),
all basis vectors positive ("caltrop")

* CCP == FCC == IVM (merging memes); a series of animations about the octet
truss featuring Bell kites, C6XTY etc.

[1] More background reading:
(good on how whole new geometries arose based on changing assumptions
regarding parallel lines)

Point your RSS reader here for a feed of the latest messages in this topic.

[Privacy Policy] [Terms of Use]

© The Math Forum at NCTM 1994-2017. All Rights Reserved.