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Find amplitude.

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

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高一必修第一册(A版).pdf · 5.6 · PDF 238 / printed page 231

Revisit first: Unit-circle coordinates and signs Identity choice and domain preservation

TOPIC 01

Amplitude, period and phase in models

Read parameters as observable changes and reconstruct models without hiding ambiguity.

What you will be able to explain

Distinguish amplitude, vertical shift, angular frequency and horizontal shift.
Reconstruct a sinusoid from sufficient observations and recognise when samples are insufficient.

Try it. Leave your reasoning visible.

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

Examples 1–3: study a full demonstration; finish a solution from its first step; then choose a method independently before opening the solution. The remaining exercises may continue after class.

01 / Foundation#Your turn

Find amplitude.

y=-2\cos3x

All inputs are real unless a smaller domain is stated.

Skills and prerequisite lessons

Scope: JM01 / JM02

Working and explanation

BUILD THE REASONING

Hint 1

Amplitude is a nonnegative distance from the midline.

Hint 2

Take the absolute value of the coefficient.

Worked solution

The range is symmetric about zero.
$-2\le y\le2$
Half the range width gives amplitude.
$(2-(-2))/2=2$

Checks and common pitfalls: The minus sign reflects the curve vertically.

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

Teacher preparation and assessment

40-minute lesson

Minutes	Focus	Teaching and student action
5	Read the displacement	Use the phase diagnostic and require factoring before choosing.
8	Separate parameters	Annotate amplitude, midline and one full cycle on a graph.
10	Reconstruct and qualify	Pairs reconstruct from extrema and discuss why phase remains free.
12	Interpret and sample	Groups solve the tide threshold and devise a measurement to distinguish sampled models.
5	Exit direction	Distinguish an upward crossing from an arbitrary zero.

Question sequence

Which measured feature fixes each parameter?
Could two different continuous curves fit all samples?

Board development

Factor before reading horizontal shift.
$\omega(x+\phi/\omega)$
Retain time units in a model.
$T=12\text{ hours}$

Expected correct and incorrect responses

“Range determines A,D but not phase” is a correct limitation.
“All readings equal, so the function is constant” overclaims the data.

Follow-up questions

What observation fixes the direction of a crossing?

Remediation

For phase errors, solve ωx+φ=0 directly, then compare with the claimed shift.

Exit check

For y=−3sin(2x)+1, give amplitude, range and least period. Expected 3, [−2,4], π.

Assessment checklist and sample responses

2 marks: parameter roles.
Full: divide phase by frequency; partial: use phase alone as shift.
2 marks: evidence and ambiguity.
Full: two fitted models plus t=1/4 distinguishing test; partial: two formulas without checking samples.