Probability and Geometry
Abstract
The activity and two discussions of this lesson connect probability and
geometry. The Polyhedra discussion leads to platonic solids, and the Probability and
Geometry discussion leads to connections between angles, areas and probability.
The subtle difference between defining probability by counting outcomes and
defining probability by measuring proportions of geometrical characteristics is
brought to light.
Objectives
Upon completion of this lesson, students will:
- have practiced calculating probability
- have seen how geometry can help solve probability problems
- have learned about platonic solids
Standards
The activities and discussions in this lesson address the following
NCTM standards:
Data Analysis and Probability
Understand and apply basic concepts of probability
- understand and use appropriate terminology to describe complementary and mutually exclusive events
- use proportionality and a basic understanding of probability to make and test conjectures about the results of experiments and simulations
- compute probabilities for simple compound events, using such methods as organized lists, tree diagrams, and area models
Geometry
Analyze characteristics and properties of two- and three-dimensional geometric shapes and develop mathematical arguments about geometric relationships
- understand relationships among the angles, side lengths, perimeters, areas, and volumes of similar objects
Use visualization, spatial reasoning, and geometric modeling to solve problems
- draw geometric objects with specified properties, such as side lengths or angle measures
- use visual tools such as networks to represent and solve problems
Links to other standards.
Student Prerequisites
- Arithmetic: Students must be able to:
- use addition, multiplication and division in solving probability problems
- work with fractions in solving probability problems
- Technological: Students must be able to:
- perform basic mouse manipulations such as point,
click and drag
- use a browser such as Netscape for experimenting with
the activities
Teacher Preparation
Students will need:
- Access to a browser
- Pencil and Paper
- Spinner Worksheet
- If desired, the Polyhedra discussion can be demonstrated with the students using:
- cardboard or plastic forms of equilateral triangles, squares, and regular pentagons
to trace on paper. If a set of pre-cut paper figures consisting of 30-40 triangles,
10-15 squares, and 15-20 pentagons is available, then forms and scissors are unnecessary.
- scissors to cut the paper
- scotch tape to put the polyhedra together
- The Spinner Game
and
the Adjustable Spinner Game require either computer
access or a set of materials for building spinners for each group of students.
- The Angles and Expected Value Discussion: from geometry to
probability discussion refers to protractors for measuring angles, so each group of students should have
a protractor.
Key Terms
This lesson introduces students to the following terms through the included discussions:
Lesson Outline
- Focus and Review
Remind students what has been learned in previous lessons that will be pertinent to this lesson and/or
have them begin to think about the words and ideas of this lesson:
- Who has ever watched the game wheel of fortune?
- Have you ever noticed when they put the $10,000 space on the wheel it is
significantly smaller than the rest of the spaces?
- Do you think size of the space affects whether or not you will land on the space?
- Objectives
Let the students know what it is they will be doing and learning today. Say something like
this:
- Today, class, we are going to begin learning about probability.
- We are going to use the computers to learn about probability,
but please do not turn your computers on until I ask you to. I want to show you a little about
this activity first.
- Teacher Input
- The Probability and Geometry discussion shows that sometimes
knowledge of geometry is required to answer probability questions.
- Guided Practice
- Have the students work with the Spinner Game and
the Adjustable Spinner Game to demonstrate probability
concepts using spinners.
Each student or group of students can construct a spinner or use the
software to construct a "virtual spinner." Conducting multiple experiments with
the spinners, students can determine experimentally the chances of selecting
each sector, and compare these chances.
If students use physical spinners, they will have to tally the results of the experiments by hand.
Each group of students can use the Spinner Experiments Table
for that.
- Using spinners, physical or virtual, from the Spinner Game
and the Adjustable Spinner Game, groups of students
can discuss how to find the exact probability of selecting each sector on their spinner, and then
compare their findings with experimental data from the Spinner Game. The following questions can help
the students:
- What features of the spinner (e.g., size, color of sectors, etc.) make a
difference for the probability, and what features do not make a difference?
- How can we decide which of the two sectors has a better chance to be
selected? Can we do it without cutting the spinner and superimposing the
sectors?
- The Polyhedra discussion connects probability and
geometry through construction of dice with various numbers of sides.
- Independent Practice
- Have students construct their own dice.
We can loosely call a die a 3-D object that can land in several
different ways when it is rolled on a flat surface. Most people are familiar
with six-sided dice. The following activities and questions can be interesting
to individual students or to groups of students:
1. Come up with a way to construct a "die" that has as many sides as you want,
starting from 3: 3, 4, 5, 100, ... Hint: pencil.
2. Using the following rules, try to construct various dice:
- You can use polygons of only one type: either equilateral triangles or
squares or regular pentagons
- Each vertex of the die has the same number of sides connected to it. In
practice, you can start from forming one vertex out of several polygons. Their
number will be dictated by geometry (3, 4 or 5 for triangles, 3 for squares, 3
for pentagons). Then attach the same number of sides to the remaining vertices,
finishing the polyhedron.
The dice that can be constructed this way are called platonic solids.
3. Can you construct a platonic solid type die out of regular hexagons? Why or
why not? Try it.
- Closure
- You may wish to bring the class back together for a discussion of the findings.
Once the students have been allowed to share what they found, summarize the results of
the lesson.
Alternate Outlines
This lesson can be rearranged in several ways.
- Have students construct spinners out of several different materials, and then compare the results
they obtain. Which materials or designs produce spinners that produce more truly "random" results?
Compare the results of many spins with these spinners with the computer-generated results from the
Spinner Game and the
Adjustable Spinner Game
to show students the advantage of using a
computer model to produce accurate results.
- Use Buffon's Needle as an
additional example of the connection between probability and geometry.
- Have groups of students read the two discussions in this lesson and prepare presentations for their
classmates that explain the content of the discussions.
Suggested Follow-Up
After these discussions and activities, the students will have an understanding of how geometry can
be used to solve probability problems. The next lesson, Conditional probability
and probability of simultaneous events, leads to a deeper consideration of the related mathematics
and to acquiring new tools for solving problems, namely the ideas and formulas connected with
conditional probability and probability of simultaneous events.
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