LEARN · EXPLAIN · REVISE

Given cos α=3/5, find cos 2α.

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

高一必修第一册(A版).pdf · 5.5 · PDF 222 / printed page 215

TOPIC 01

Trigonometric identities and transformations

Build understanding of trigonometric identities and transformations through definitions, contrasting cases and justified applications.

Use trigonometric identities and transformations with explicit angle units and domains.
Connect representations and justify the steps, including boundary cases.
Explain a solution and apply the idea to a changed situation.

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

01 / Foundation#Your turn

\cos\alpha=3/5

Skills and prerequisite lessons

Working and explanation

BUILD THE REASONING

Hint 1

Choose the double-angle form involving cosine alone.

Hint 2

Square the given cosine before doubling.

Worked solution

Choose the double-angle form involving cosine alone.
Calculate or simplify this relation.
$\cos2\alpha=2\cos^2\alpha-1=18/25-1=-7/25$
The answer need not have the same sign as cos α.

The requested value is -0.28.

Checks and common pitfalls: The answer need not have the same sign as cos α.

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

Teacher preparation and assessment

What must be true before using the main rule for trigonometric identities and transformations?
Which representation makes this task easier, and why?
Change one assumption. Does the conclusion survive?

Use trigonometric identities and transformations with explicit angle units and domains.
Angle sums: Use paired sine and cosine products in the addition identities.
$\sin(a+b)=\sin a\cos b+\cos a\sin b$
Equivalent forms: Choose a double-angle or half-angle form matching the given data.
Close with: conditions → representation → reasoning → check.

Expected reasoning: Use paired sine and cosine products in the addition identities.
Expected reasoning: Choose a double-angle or half-angle form matching the given data.
Expected correction: Cancelled factors cannot restore originally forbidden inputs.