cheesemonkey wonders

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Saturday, February 8, 2014

Arithmetic of Complex Numbers Placemat Activity - Algebra 2 + Complex Instruction (CI)

Just because you have an all-groupwork and all-Complex Instruction (CI) format doesn't mean you don't need practice activities too.

Our Algebra 2 kids were getting the concepts of complex numbers and complex conjugates, but were still kinda shaky in terms of fluency in working with them.

Based on ideas I stole borrowed a long time ago from the fabulous Kate Nowak (@k8nowak, and the equally fabulous Rachel Kernodle (@rdkpickle, ), I proposed a placemat activity to my ever-game Algebra 2 teaching team and they dove right in.

We have typical CI four-person table teams set up in each of our rooms, with each person assigned a specific role based on where they're seated at the table. Our roles are Facilitator, Resource Manager, Recorder/Reporter, and Team Captain, although of course, your mileage may vary. Each role has specific tasks they are expected to perform; for example, only the Resource Manager may call the teacher over for a group check-in or a group question (in our program, teachers only accept and answer group questions).

Each table was given:
  • two, double-sided "placemat" sheets for doing work in the center of the table
  • a set of problem cards (there are four sets, one for each round of play; to simplify clean-up and organization, I printed each round of cards single-sided on a different color of paper, one set per table group. I've got 7 tables in my room, so I made seven sets of cards. I laminated them and clipped them together, but hey, that's just me)
  • the sum to which all four answers for any given round should add up
The sum for each round was written on the whiteboard, though it could have been projected via document camera or Keynote/Powerpoint slide.

We had mathematical objectives for the activity as well as CI or norms-based, group work objectives. My students in particular needed reinforcement in group work norms and collaboration. Our objectives were:

     Math Objectives

  • achieve greater fluency in the arithmetic of complex numbers (including the distributive property)
  • deepen understanding of and fluency with the powers of i
  • deepen understanding of and fluency with complex conjugates

     Group Work Objectives

  • work in the middle of the table
  •  same problem, same time (no one moves on until everyone moves on)
  • using table group members as resources

The next time I run this activity, I will definitely give a Participation Quiz because the group work norms are so beautifully reinforced in this activity.

How We Ran It
Recorder/Reporter writes the sum in the central oval of the first side of the placemat. Each group member gets a problem card for round 1 (problem a) and works his or her problem on his or her quadrant of the placemat.

When everybody is finished with their problem, the Facilitator facilitates the addition of all four answers. If they add up to the given sum for that round, the Resource Manager calls the teacher over for a "checkpoint" and the next set of cards for the subsequent round of work.

If their answers don't add up to the given sum, they need to work together through everybody's work on the placemat to diagnose what went wrong and where, as well as how to fix it. Then when they've fixed it, they call the teacher over for a checkpoint and the next set of cards for the subsequent round.

Group Work Benefits — Reinforcing Norms
For my classes, the greatest benefits of this activity came from the fact that it forced students to work in the middle of the table, to use each other as resources, and to talk mathematics. Getting kids to work in the middle of the table is the hardest part of CI, in my view, because it goes against the grain of most of their in-school conditioning. The placemat format makes it nearly impossible NOT to work in the middle of the table. And once they're doing that, it seemed like everything else ran pretty smoothly.

I especially liked the fact that this activity created a context in which students experienced an intellectual need for the using the rules of arithmetic for complex numbers and for the powers of i. It was situationally motivated, but extremely targeted.

Sums for Each Round 
The sums for each round are as follows (if you find an error, please speak up):
  • Round 1 (problem a):   26-73i
  • Round 2 (problem b):  0
  • Round 3 (problem c):  165
  • Round 4 (problem d):  2 – 48i
PDF Files for the activity
These are available also on the Math Teacher Wiki on the Algebra 2 page. If you haven't visited the Math Teacher Wiki, you don't know what you're missing.


  1. Just used this today in my Algebra II class and it worked great! Thanks for the awesome resource!!

  2. These look great! I am going to try these in my class next week.

  3. I thought I had found the perfect activity but the link takes me to a disabled page. Are they available anywhere else?

    1. Sorry, our wiki got hacked. Here's a new, working link to the file:

      - Elizabeth (@cheesemonkeysf)