Imaginary Numbers Day 1 of 2

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Students will be able to define imaginary numbers and use them to perform operations.

Big Idea

Repeated reasoning is a central feature in this lesson.

Warm Ups

10 minutes

I include Warm ups with a Rubric as part of my daily routine. My goal is to allow students to work on Math Practice 3 each day. Grouping students into homogeneous pairs provides an opportunity for appropriately differentiated math conversations. This lesson’s Warm Up- Imaginary Numbers Day 1, asks students to describe what makes a number real as well as how they would know that a number isn't real.

I also use this time to correct and record any past Homework.

Imaginary Numbers

37 minutes

I am beginning this lesson with an area model just like the previous lesson.  We start by discussing the side length of a square with area 1cm2 and then talk about another with an area of -1cm2.  This discussion leads us to the definition of imaginary numbers, both i2 = -1 and i = √-1. 

We then extent this to i3 and i4.  After students have discussed how they found equivalent expressions for these cases, we extend the pattern to i15 and then i123 (Math Practice 8).  This will be a challenge (Math Practice 1).  As students work, I walk around writing the variations I see on a small whiteboard which I then transfer to the full whiteboard.  The students then discuss the variations until we come up with an solution that everyone can agree with. 

Next, I ask students to write a summary which will help them formalize their thoughts on this repeated reasoning activity.  Once the students have written their summary, I group them into fours(usually by seating) and have them share summaries with each other.  They then pick their favorite of the four summaries and these will be shared with the class.

These are several additional examples that can be included or left out depending on the proficiency of the class.


The goal of this short assignment is to ensure that students have a solid understanding of imaginary number pattern explored in class and included an extension problem that I talk about in my video narrative.

Exit Ticket

3 minutes

I use an exit ticket each day as a quick formative assessment to judge the success of the lesson.  

Today's exit ticket asks students to multiply two imaginary numbers which specifically checks whether students truly understand that that i2 = -1.