# Engaging students: Introducing variables and expressions

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission again comes from my former student Christine Gines. Her topic, from Pre-Algebra: introducing variables and expressions.

APPLICATIONS

As we all know, introducing variables in a Mathematics class often intimidates students. As teachers, we can minimize this by creating activities where students are eased into the new topic in a fun and educational environment.  This can be achieved through the following activity that introduces variables:

In this activity, students discover “the value of words.” On notebook paper, have students write the letters of the alphabet in order down the left side of the paper. Down the right side of the notebook paper, have them write the numbers from 0 to 25. The letters should corresponding to the numbers. The numbers are the values to each letter, or variable.

To begin, you could have your students find the value of their own name and last name.

Ex. Chris –> C=2, H=7, R=17, I=8, S=18

= 2+7+17+8+18 = 52

You could ask the following questions:

• Which has a higher value – first or last name?
• What is the difference in the values of your first and last names?
• Find words whose values are equal to 25, 36, or 100.
• What is the three-letter word with the greatest value?
• Are the greatest values always associated with words that contain the most letters?

You could also pair your students and have them write codes to each other. Furthermore, challenge them to write their code with value restrictions and allowing them to *,/,+,-.

This activity develops algebraic thinking in a concrete manner students can understand without presenting them with an overwhelming amount of new information. It is a very flexible activity in which you could make it your own and get the kids excited about it. For example, the activity could even be competitive by challenging students to write an expression for CAT where the value would equal 2 (C+A*T = 2+0*10). This is definitely something I would use to introduce variables.

CURRICULUM

A variable expression is a combination of variables, numbers and operations. The only new information being presented is the unknown represented as variables and how to solve for that variable. Students don’t know this, but it’s quite similar to what they have been doing in school for years. Take 2x=4 for example. We know x=2 because 4/2=2. This expression is equivalent to just writing 4/2=_, which is a simple division problem that students have seen time and time again.

Variable expression are not always given, though. Students will learn how to construct them by analyzing word problems for key clues. This is where the vocabulary students have been working with comes into play.  Common words that they will see are sum, difference, quotient, product, etc.

A key rule to +/- fractions is “Whatever you do to the top, you have to do to the bottom.” This theme directly correlates with solving expression with respect to the left and right side of the equal sign. Therefore, we can conclude that variable expressions are a combination of skills that students have learned previously with the exception of written variables.

TECHNOLOGY

With the fast growth of  technology, more and more useful sources are becoming readily available to us and it’s important to take advantage of this. Math Play is a website that provides a variety of interactive online games organized by content and all grade levels.

One game in particular, Algebraic Expressions Millionaire Game, serves perfectly as an introduction to constructing variable expressions. The game has the theme of “Who wants to be a millionaire?” and challenges students to chose an equivalent representation of an expression written in words. The problems increase in difficulty as you progress, using clues such as less than, difference, sum, product, quotient, etc.  This Algebraic Expressions Millionaire Game can be played online alone or in two teams. The link to this game can be found below:

http://www.math-play.com/Algebraic-Expressions-Millionaire/algebraic-expressions-millionaire.html

This game is a great way for students to develop a conceptual idea of what variable expressions represent. It also builds a foundation for solving and constructing word problems. Try pairing students to compete against each other to add motivation. You could even hold a tournament!

# Engaging students: Introducing variables and expressions

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission again comes from my former student Caitlin Kirk. Her topic, from Pre-Algebra: introducing variables and expressions.

To keep track of some of the coldest things in the universe, scientist use the Kelvin temperature scale that begins at 0 Kelvin, or Absolute Zero. Nothing can ever be colder than Absolute Zero because at this temperature, all motion stops. The table below shows some typical temperatures of different systems in the universe.

Table of Cold Places

 Temp.(K) Location 183 Vostok, Antarctica 160 Phobos- a moon of Mars 128 Europa in the summer 120 Moon at night 88 Miranda surface temp. 81 Enceladus in the summer 70 Mercury at night 55 Pluto in the summertime 50 Dwarf Planet Quaoar 33 Pluto in the wintertime 1 Boomerang Nebula 0 ABSOLUTE ZERO

You are probably already familiar with the Celsius (C) and Fahrenheit (F) temperature scales. The two formulas below show how to switch from degrees-C to degrees-F.

$C = \frac{5}{9} (F-32)$

$F = \frac{9}{5} C + 32$

Because the Kelvin scale is related to the Celsius scale, we can also convert from Celsius to Kelvin (K) using the equation:

$K = 273 + C$

Problems

Use these three equations to convert between the three temperature scales:

Problem 1: 212 F converted to K

Problem 2: 0 K converted to F

Problem 3: 100 C converted to K

Problem 4: Two scientists measure the daytime temperature of the moon using two different instruments. The first instrument gives a reading of +107 C while the second instrument gives +221 F.

a. What are the equivalent temperatures on the Kelvin scale?

b. What is the average daytime temperature on the Kelvin scale?

Problem 5: Humans can survive without protective clothing in temperatures ranging from 0 F to 130 F. In what, if any, locations from the table above can humans survive?

Solutions

Problem 1: First convert to C:  C = 5/9 (212-32) = +100 C. Then convert from C to K: K = 273 + 100 = 373 Kelvin.

Problem 2: First convert to Celsius:    0 = 273 + C so C = -273. Then convert from C to F: F = 9/5 (-273) + 32 = -459 Fahrenheit.

Problem 3: K = 273 – 100 = 173 Kelvin.

Problem 4:

a. 107 C becomes K = 273 + 107 = 380 Kelvin.  221 F becomes C = 5/9(221-32) = 105 C, and so K = 273 + 105 = 378 Kelvin.

b. (380 + 378)/2 = 379 Kelvin

Problem 5:

First convert 0 F and 130 F to Celsius so that the conversion to Kelvin is quicker. 0 F becomes C = 5/9(0-32) = -18 C (rounded to the nearest degree) and 130 F becomes C = 5/9 (130-32) = 54 C (rounded to the nearest degree).

Next, convert -18 C and 54 C to Kelvin. -18 C becomes K = 273-18 = 255 and 54 C becomes k = 273 + 54 = 327 K.

None of the locations on the table have temperatures between 255 K and 327 K, therefore humans could not survive in any of these space locations.

A. How can this topic be used in your students’ future courses in mathematics or science?

This topic is one of the first experiences students have with algebra. Since algebra is the point from which students dive into more advanced mathematics, this topic will be used in many different areas of future mathematics. After mastering the use of one variable, with the basic operations of addition, subtraction, multiplication, and division, students will be introduced to the use of more than one variable. They may be asked to calculate the area of a solid whose perimeter is given and whose side lengths are unknown variables. Or in a more advanced setting, they may be asked to calculate how much money will be in a bank account after five years of interest compounded continuously. In fact, the use of variables is present and important in every mathematics class from Algebra I through Calculus and beyond. There very well may never be a day in a mathematics students’ life where they will not see a variable after variables have been introduced.

B.  How does this topic extend what your students should have learned in previous courses?

In basic arithmetic, probably in elementary or early middle school math classes, students learn how to do calculations with numbers using the four basic operations of addition, subtraction, multiplication and division. They also learn simple applications of these basic operations by calculating the area and perimeter of a rectangle, for example. Introducing variables and expressions is a continuation of those same ideas except that one or more of the numbers is now an unknown variable. Students can rely on the arithmetic skills they already possess when learning this introduction to algebra with variables and expressions.

Students are familiar with calculating the area and perimeter of figures like the one on the left before they are introduced to variables. Later, they may see the same figure with the addition of a variable, as shown on the right. The addition of the variable will come with new instructions as well.

The difficulty of problems using variables is determined by the information given in the problems. For instance, the problem on the right can be a one step equation if an area and perimeter are given so that students only need to solve for w. The difficulty can be increased by giving only a perimeter so that students must solve for w and then for the area.