LEARN · EXPLAIN · REVISE

Find sin²θ for a cube’s space diagonal making angle θ with the base.

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

高一必修第二册(A版).pdf · 8.6 · PDF 153 / printed page 146

TOPIC 01

Perpendicular lines and planes in space

Build understanding of perpendicular lines and planes in space through definitions, contrasting cases and justified applications.

Use definitions, valid spatial reasoning and metric checks for perpendicular lines and planes in space.
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

Skills and prerequisite lessons

Working and explanation

BUILD THE REASONING

Hint 1

Use perpendicularity tests with intersecting directions, then identify the relevant projection or section.

Hint 2

Use vertical height over diagonal length.

Worked solution

Use perpendicularity tests with intersecting directions, then identify the relevant projection or section.
Calculate or simplify this relation.
$\sin\theta=\frac{a}{a\sqrt3}=\frac1{\sqrt3};\quad\sin^2\theta=1/3$
The cube size cancels, so the angle is scale-independent.

The requested value is 0.33333333.

Checks and common pitfalls: The cube size cancels, so the angle is scale-independent.

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

Teacher preparation and assessment

What must be true before using the main rule for perpendicular lines and planes in space?
Which representation makes this task easier, and why?
Change one assumption. Does the conclusion survive?

Use definitions, valid spatial reasoning and metric checks for perpendicular lines and planes in space.
Line-plane test: Perpendicularity to two intersecting in-plane lines proves perpendicularity to the plane.
Angles and distances: Use orthogonal projections for line-plane angles and perpendicular sections for dihedral angles.
Close with: conditions → representation → reasoning → check.

Expected reasoning: Perpendicularity to two intersecting in-plane lines proves perpendicularity to the plane.
Expected reasoning: Use orthogonal projections for line-plane angles and perpendicular sections for dihedral angles.
Expected correction: Perpendicular planes do not make every contained line perpendicular to the other plane.