standard_description: Fluently add, subtract, multiply, and divide multi-digit decimals using the standard algorithm for each operation.
problem_activity: Warm-up
The purpose of this activity is to focus students' attention on the two parts of a complex number: the real part and the imaginary part. This will be useful in the Information Gap, when students will need to be specific about the information they want and to strategize about how to use it (MP1).
Launch
Display the following for all to see:
\((a+bi)-(\underline{\phantom{00}} + \underline{\phantom{00}}i) = \text?\)
Ask, "In order for the result of this subtraction to be a real number, what has to be true about the missing imaginary part of the complex number subtracted from
\(a+bi\)
?" (It has to match the imaginary part of the other expression so it will sum to 0, which means that
\(b\)
must go in the blank.) After quiet think time, invite students to share their thoughts with a partner. Then invite partners to share with the class.
Student Facing
Here are some complex numbers with an unknown difference:
\((10+4i)-(\underline{\phantom{00}} + \underline{\phantom{00}}i) = \text?\)
If the result of this subtraction is a real number, what could the second complex number be?
If the result of this subtraction is an imaginary number, what could the second complex number be?
Student Response
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Activity Synthesis
Ask students to share the expressions they made by filling in the blanks. Highlight what is common about the responses to each question (all responses to the first question have the same imaginary part, and all responses to the second question have the same real part).
Tell students that in the next activity they will continue to reason about the real and imaginary parts of complex numbers based on what they know about how the numbers are combined.


aligns: no