LEARN · EXPLAIN · REVISE

Evaluate sin 225° exactly.

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

高一必修第一册(A版).pdf · 5.3 · PDF 195 / printed page 188

TOPIC 01

Reduction formulas

Build understanding of reduction formulas through definitions, contrasting cases and justified applications.

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

01 / Foundation#Your turn

\sin225^{\circ}

Skills and prerequisite lessons

Working and explanation

BUILD THE REASONING

Hint 1

Identify the reference angle and quadrant.

Hint 2

The angle is in quadrant III.

Worked solution

Identify the reference angle and quadrant.
Calculate or simplify this relation.
$\sin(180^{\circ}+45^{\circ})=-\sin45^{\circ}=-\sqrt2/2$
Sine is negative in the lower half-plane.

−√2/2.

Checks and common pitfalls: Sine is negative in the lower half-plane.

Reasoning checklist · self / teacher assessment

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

Teacher preparation and assessment

Use reduction formulas with explicit angle units and domains.
Symmetry: Reflections determine which coordinate changes sign.
Periodicity: A full turn preserves sine and cosine; a half turn preserves tangent when defined.
Close with: conditions → representation → reasoning → check.

Expected reasoning: Reflections determine which coordinate changes sign.
Expected reasoning: A full turn preserves sine and cosine; a half turn preserves tangent when defined.
Expected correction: A reflected cosine often changes sign.