Kenneth ChengMathematical modeling
← Senior Mathematics Studio

LEARN · EXPLAIN · REVISE

Find the real part of the product.

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 real part of the product.

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

Working and explanation

BUILD THE REASONING

Hint 1
Expand with i²=−1 and use a conjugate to remove a complex denominator.
Hint 2
The product i(−i) equals +1.
Worked solution

  1. Expand with i²=−1 and use a conjugate to remove a complex denominator.

  2. Calculate or simplify this relation.

    (9+i)(2−i)=19+−7i(9+i)(2-i)=19+-7i

  3. Separate the real terms from the coefficient of i.

The requested value is 19.

Checks and common pitfalls: Separate the real terms from the coefficient of i.

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 ↗