Tag: squares

Posted in Geometry

If One World Trade Center is a prism and not an antiprism, would it be less in volume?

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One World Trade Center, more simply known as 1 WTC and previously known as the Freedom Tower, is the lead building of the new World Trade Center complex in Lower Manhattan, New York City. The supertall skyscraper is 104 storey  and is being constructed in the northwest corner of the 16-acre World Trade Center site. The image at the right shows the design as of May 2012.

One World Trade Center is an example of an antiprism. The square edges of the world trade centre tower’s cubic base are chamfered back, transforming the building’s shape into an elongated square antiprism with eight tall isosceles triangles—four in upright position and another 4 in upside down direction. Near its middle, the tower forms a perfect octagon, and then culminates in a glass parapet whose shape is a square oriented 45 degrees from the base. My question is Is this bigger than if it were a square prism? How about in terms of surface area?

Antiprisms are similar to prisms except the bases are twisted relative to each other, and that the side faces are triangles, rather than quadrilaterals. Here’s a model of a square antiprism.

square antiprism

The surface area of the antiprism may be bigger than the prism because of the additional faces but I’m not quite sure. How about the volumes? Cavalieri’s principle does not apply because the cross sections do not have the same area. Anyone wants to resolve and explain this? There is a formula for volumes and surface areas of antiprisms but I’d appreciate a more intuitive solution.

Note: All information and images about 1 WTC is from Wikipedia. Square antiprism model from eusebeia.dyndns.org.

Posted in Algebra, Geogebra, Geometry

The Pythagorean Theorem Puzzle

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Math puzzles are great activities for enjoying and learning mathematics. The following is an example of Tachiawase. Tachiawase is a popular puzzle in Japan which involve dissecting a geometrical figure into several parts and then recombining them to form another geometrical figure. The puzzle below is credited to Hikodate Nakane (1743). This was one of the puzzles distributed at the booth of Japan Society of Mathematical Education during the ICME 12 in Seoul this year.

Make a shape that is made from two different sizes of squares by dividing them into three parts  then recombine them into one square. [Reformulated version: Make two cuts in the shapes below to make shapes that can be recombined into a bigger square.]

two squares puzzle

Here’s how I figured out the puzzle: I know that it must have something to do with Pythagorean Theorem because it asks to make a bigger square from two smaller ones. But where should I make the cut? I was only able to figure it out after changing the condition of the puzzle to two squares with equal sizes. It reduced the difficulty significantly. This gave me the idea where I could make the cut for the side of the square I will form. The solution to this puzzle also gave me an idea on how to teach the Pythagorean theorem.

I made the following GeoGebra mathlet (a dynamic math applet) based on the solution of the puzzle. I think the two-square math puzzle is a little bit tough to start the lesson so my suggestion is to start the lesson with this mathlet and then give the puzzle later.  As always, the key to any lesson are the questions you ask. For the applet below, here’s my proposed sequence of questions:

  1. What are the areas of each of the square in the figure? Show at least two ways of finding the area.
  2. How are their areas related? Drag F to find out if your conjecture works for any size of the squares.
  3. Can you think of other ways of proving the relationships between the three squares without using the measures of the sides?
  4. If the two smaller squares BEDN and GFNH have sides p and q, how will you express the area of the biggest square LEJG in terms of the area of the smaller ones?
  5. Express the length of the sides LEJG in terms of the sides of BEDN and GFNH.

[iframe https://math4teaching.com/wp-content/uploads/2012/08/Pythagorean.html 550 450]

After this lesson on Pythagorean relation you can give the puzzle. Once they have the correct pieces, ask the students to move the pieces using transformation in the least possible moves. They should be able to do this in three moves using rotation. Click here to download the applet. Note: If you don’t see the applet, enable java in your browser

Use the comment sections to share your ideas for teaching the Pythagorean relation. If you like this post, share it to your network. Thank you.

Posted in Algebra, GeoGebra worksheets

What is a coordinates system?

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This is the first in the series of posts about teaching mathematics and Geogebra tools at the same time. I’m starting with the most basic of the tools in GeoGebra, the point tool. What would be a better context for this than in learning about the coordinate system. Teacher can use the following introduction about geographic coordinates system and the idea of number line as introduction to this activity.

A coordinates system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of a point or other geometric elements. An understanding of coordinate system is very important. For example, a geographic coordinate system enables every location on the Earth to be specified by a set of numbers. The coordinates are often chosen such that one of the numbers represent vertical position, and two or three of the numbers represent horizontal position. A common choice of coordinates is latitude and longitude. Sometimes, a third coordinate, the elevation is included. For example:

Philippine  Islands are located within the latitude and longitude of 13° 00 N, 122° 00 E. Manila, the capital city of Philippines is 14° 35′ N, 121º 00 E’.

In mathematics we study coordinates systems in order to describe location of points, lines and other geometric elements. The numberline is an example of a coordinates system which describe the location of a point using one number. The coordinates of a point on a numberline tells us the location of a point from zero. But what if the point is not on the line but above of below it? How can we describe exactly the location of that point? This is what this activity is about: how to describe the position of points on a plane.

You would need to familiarize your students first about the GeoGebra window shown below before asking them to work on the GeoGebra worksheet.

Click here to go the GeoGebra worksheet – What are coordinates of points?

 

Posted in Algebra, Assessment

Assessing understanding via constructing test items

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Assessing understanding of mathematics can also be done by asking students to write test items.  Here’s my favorite assessment item. I gave this to a group of teachers.

Possible  answers/ questions.

Year level: Third year (Year 9)

Question 1 – What is the distance of P from the origin?

Question 2 – What is the area of circle P with radius equal to its distance from the origin?

Question 3 – With P as one of the vertex, draw square with area 2 square units.

Year level: Second year (Year 8 )

Question 1 – Write the equation of the line that passes through P and the origin.

Question 2 – Write 3 equations of lines passing through (2,1).

Question 3 – Write the equation of the family of lines passing through (2,1).

Year level: First year (Year 7)

Question 1 – What is the ordinate of point P?

Question 2 – Locate (-2, 1). How far is it from P?

Question 3 – Draw a square PQRS with area 9 square units. What are the coordinates of that square?

How about using this exercise to assess your students? Ask them to construct test items instead of asking them to answer questions.

Here are a few more assessment items which I constructed based on the TIMSS Framework:

  1. Trigonometric Functions
  2. Zeroes of Functions
  3. Graphs of Rational Functions