July 9th, 2015 by Dan Meyer
This Week’s Skill
Determining if a relationship is a function or not.
A relationship that maps one set to another can be confusing. Questions like, “What single element does 2 map to in the output set below?” are impossible to answer because 2 maps to more than one element.
By contrast, a function is a relationship with certainty. Take any element of the input and ask yourself, “Where does this function say that element maps?” You aren’t confused about any of them. Every input element maps to exactly one element in the output.
Pearson and McGraw-Hill’s Algebra 1 textbooks simply provide a definition of a function. Pearson’s definition refers to a previous worked example. McGraw-Hill has students apply the definition to a worked example immediately afterwards. Khan Academy dives straight into an abstract explanation of the concept. In none of these cases is the need for functions apparent. Students are given functions without ever feeling the pain of not having them.
What a Theory of Need Recommends
If we’d like students to experience the need for the certainty functions offer us, it’s helpful to put students in a place to experience the uncertainty of non-functional relationships first. Here is what I’m talking about.
Put the letters A, B, C, and D on your back wall, spaced evenly apart.
Ask every student to stand up. Then give them a series of instructions.
If you walked to school today, stand under A.
If you rode your bike to school today, stand under B.
If you drove or rode in a vehicle today, stand under C.
If you got to school any other way, stand under D.
If it took you fewer than 10 minutes to get to school today, stand under B.
If it took you 10 or more minutes to get to school today, stand under D.
If you’re in seventh grade, stand under A.
If you’re in eighth grade, stand under B.
If you’re in ninth grade, stand under C.
If you’re in any other grade, stand under D.
These instructions are all clear and easy to follow. Students are certain where they should go. Then give two other sets of instructions.
If you’re wearing blue, stand under A.
If you’re wearing red, stand under B.
If you’re wearing black, stand under C.
If you’re wearing white, stand under D.
If you were born in January, stand under A.
If you were born in February, stand under B.
If you were born in March, stand under C.
If you were born in April, stand under D.
Perhaps you see how these last two examples generate a lack of certainty. Students were lulled by the first examples and may now feel a headache.
“I’m wearing white and red. Where do I go?”
“I was born in August. There’s no place for me to stand.”
Now we gather back together and apply formal language to the concepts we’ve just felt. “Mathematicians call these three relationships ‘functions.’ Here’s why. Why do you think these relationships aren’t functions?” Invite students to interrogate the concept of a function in different contexts. Try to keep the focus on certainty – can you predict the output for any input with certainty? – rather than on the vertical line test or other rules that expire.
Next Week’s Skill
Graphing linear inequalities. It’s extraordinarily easy to turn questions like “Graph y < -2x + 5" into the following series of steps:
- Graph the line.
- If the inequality includes the boundary, make the line solid. Otherwise, make the line dashed.
- Test a point on either side of the line. Use (0,0) if possible.
- If that point is a solution to the inequality, shade that side of the line.
- If that point isn’t a solution to the inequality, then shade the other side of the line.
Students can become quite capable at executing that algorithm without understanding its necessity or how it figures into algebra’s larger themes.
What can you do with this?
Kate Nowak encouraged me to look at other textbooks beyond McGraw-Hill and Pearson’s. She recommended CME, which, it turns out, does some great work highlighting this need for functions. It asks students to play a “guess my rule” game, one which has a great deal of certainty. Each input corresponds to exactly one output. Then the CME authors offer a vignette where a partner reports multiple outputs for the same input, making the game impossible to play. Strong work, CME buds.