The investigation for the day has two different levels, which is set up to give you some flexibility in how you use this time. There is a good chance that some students will still be working on the portfolio tasks from the first two days (Develop an Algorithm and Lattice Points on Circles). If this is the case, I think it is more effective to give them time to continue with those tasks, while other students get started writing the circle equations. During the warm-up, I get a quick idea about this: any students who need a lot of help with problem (1), (2) and (3) may take more time to work on the tasks from Day 1 and 2, while students who figure out problem (4) can get started on today's tasks.
The goal for today is that every student master Write Equations for Circles Level 1. So once I see that the majority of students have mastered the first 3 problems, I tell students their options: They can continue working on the two tasks from the two previous days, or they can transition to a new task based on problem (4.) I tell them that everyone does need to learn what is going on in problem (4) because that is the key learning necessary to complete Write Equations for Circles Level 1. Students can transition when they are ready.
Letting students decide on their own transition times does open things up to a bit of chaos—and it gets better over time the more I coach students to make these decisions effectively. This means that I circulate constantly during this time and ask students either about the content or about their choice about which tasks to work on.
When I talk to students about their choice of task, it sounds like this:
As students get started on the Writing Equation task, the big question is:
What number sentence will be true for any point (x, y) that lies on the circle?” In order for students to explore this, ask them to find a lattice point on the circle and show the steps they would use to prove that it is on the circle. These steps basically turn into the equation. This is a good opportunity to use the graphing calculator desmos.com/calculator for automatic feedback: if they find an equation, type it into desmos and see if it generates a circle that fits the requirements. You can make this optional, and present it to students by saying, “How could you use Desmos to check your own answer?
The Writing Circle Equations Level 2 problems are much more challenging and will require that students recall (or understand) two big ideas from geometry:
I give student some hints to help them develop these ideas. First, I frame them as questions:
I give students the chance to come up with these ideas on their own. Even if they don’t figure them out and you end up telling them, it is still a big stretch to translate these geometric ideas to the coordinate plane. So this should be a good challenge for them.