LEARN · EXPLAIN · REVISE

Find the real part of the product.

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

高一必修第二册(A版).pdf · 7.2 · PDF 82 / printed page 75

TOPIC 01

Arithmetic of complex numbers

Build understanding of arithmetic of complex numbers through definitions, contrasting cases and justified applications.

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

01 / Standard#Your turn

(4+i)(2-i)

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

The requested value is 9.

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

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

Teacher preparation and assessment

Connect geometric and algebraic forms of arithmetic of complex numbers.
Multiplication: Expand algebraically and replace i² with −1.
Division: A conjugate turns a nonzero complex denominator into its positive squared modulus.
Close with: conditions → representation → reasoning → check.

Expected reasoning: Expand algebraically and replace i² with −1.
Expected reasoning: A conjugate turns a nonzero complex denominator into its positive squared modulus.
Expected correction: The sign from i² must not be lost.