# Which One Doesn’t Belong?

As part of an upcoming “TNIFHS” post, I wanted to include one example of what Christopher Danielson’s approach to a better shapes book might look like in a high school math class. [Update: Christopher’s Which One Doesn’t Belong? is available from Stenhouse; from Pembroke for Canadians.] But then I had some fun with this and created a few more. In each set, a reason can be given for each of the four options being the odd one out. I’ve done this type of thing with numbers and equations before. Worthwhile, but not what I’m going for here. Each set below is pictorial. Also, I went naked; I stripped the graphs of grid lines and ordered pairs. More noticing properties, less determining equations. Aside from graphs, where else in secondary mathematics might this fit? The last two images below are my attempts at answering this question.

Update: Mary Bourassa created a website.

Update (10/04/17):

# One of these things is not like the others

When you read the title of this post, did you think Sesame Street? Foo Fighters? Or, like me, both?

Recently, Geoff shared seven (sneaky) activities to get students talking mathematically. One activity, ‘odd one out’, involves having students pick the one mathematical thing that doesn’t belong. This reminds me of one strategy used by Dr. Marian Small to create open questions – asking for similarities and differences.

Here’s my ‘odd one out’ question:

Which of the following quadratic functions doesn’t belong? (Dr. Small might ask “Which of these four functions are most alike?”)
$y=2\left( x-1\right) ^{2}+3$
$y=\dfrac {1} {2}\left( x-3\right) ^{2}-5$
$y=3\left( x+2\right) ^{2}-4$
$y=-\dfrac {3} {2}\left( x-4\right) ^{2}+6$

Students might say,
$y=2\left( x-1\right) ^{2}+3$ because it does not cross the x-axis
$y=\dfrac {1} {2}\left( x-3\right) ^{2}-5$ because it is a vertical compression of y = x²
$y=3\left( x+2\right) ^{2}-4$ because it is a horizontal translation to the left
$y=-\dfrac {3} {2}\left( x-4\right) ^{2}+6$ because it opens down

Do the graphs of these functions strengthen your choice or make you change your mind?

I carefully chose the values of ap, and q in y = a(x – p)² + q so that students could reasonably argue that any one of the functions could be picked as the odd one out. Because I am not looking for one particular answer, each student should be able to confidently answer the question and contribute to a mathematical discussion. Planning disagreement is key; it means students will have to justify their mathematical thinking.

Sneaky.