LEARN · EXPLAIN · REVISE

Find the coefficient of x² in (1−2x)^m.

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

高三選擇性必修第三册(A版).pdf · 6.3 · PDF 34 / printed page 29

TOPIC 01

Binomial theorem

Use the general term to locate coefficients, constants and coefficient sums.

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

01 / Standard#Your turn

m=13

Skills and prerequisite lessons

Working and explanation

BUILD THE REASONING

Hint 1

Decide whether choices are alternative cases or successive stages, then check overlap.

Hint 2

Use this intermediate relation.

C_m^2(-2)^2

Worked solution

Decide whether choices are alternative cases or successive stages, then check overlap.
Apply the stated relation and retain its conditions.
$[x^2]=4C_{13}^2=312$
An even selection index makes this coefficient positive.

The requested value is 312.

Checks and common pitfalls: An even selection index makes this coefficient positive.

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

Teacher preparation and assessment

Defining relation: Use the general term to locate coefficients, constants and coefficient sums.
$(a+b)^n=\sum_{r=0}^nC_n^ra^{n-r}b^r$
Conditions: The exponent n is a nonnegative integer; term number is r+1, not r.