Kenneth ChengMathematical modeling
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Find the sum a+b when the complex number is zero.

Read the idea, work independently, then explain what changed.

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TOPIC 01

Complex numbers: mixed assessment

A mixed assessment: identify the method, justify it and revise your reasoning.

What you will be able to explain

  • Connect the chapter skills without relying on the order of the exercises.

Try it. Leave your reasoning visible.

Use one hint at a time. A correction explains what changed, not just the final answer.

01 / Standard#Your turn

Find the sum a+b when the complex number is zero.

(a−9)+(b+2)i=0(a-9)+(b+2)i=0
  • Use the domain, units and sampling assumptions stated in the question.
Skills and prerequisite lessons

Working and explanation

BUILD THE REASONING

Hint 1
Use z=a+bi with real a,b and i²=−1; distinguish parts from terms.
Hint 2
Both real and imaginary parts must vanish.
Worked solution

  1. Use z=a+bi with real a,b and i²=−1; distinguish parts from terms.

  2. Calculate or simplify this relation.

    a=9,b=−2⇒a+b=7a=9,\quad b=-2\Rightarrow a+b=7

  3. One zero part alone does not make the complex number zero.

The requested value is 7.

Checks and common pitfalls: One zero part alone does not make the complex number zero.

Think first. Reveal a hint when the class is ready.

Focus on one question

Teacher preparation and assessment

Question sequence

  • Ask students to name the relevant condition before calculating.

Board plan

  • Compare valid methods and annotate their conditions.

Anticipated thinking

  • A correct final value may still hide a missing assumption.

Assessment checklist

  • Check the method, conditions, reasoning and interpretation separately.

No sign-in. Work stays in this browser. Export before clearing browser data. Written reasoning is assessed with a checklist.

Curriculum and source notes ↗