LEARN · EXPLAIN · REVISE

A bag has t red and 3 blue balls; select 2 without replacement. Find the probability both are blue.

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

高三選擇性必修第三册(A版).pdf · 7.4 · PDF 77 / printed page 72

TOPIC 01

Binomial and hypergeometric distributions

Choose the correct model for repeated independent trials or sampling without replacement.

Choose the correct model for repeated independent trials or sampling without replacement.
Justify the method and check the conditions in a new situation.

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

01 / Standard#Worked example

t=3

Binomial trials have fixed n, common p and independence; hypergeometric sampling uses a finite population without replacement.

Skills and prerequisite lessons

Working and explanation

BUILD THE REASONING

Hint 1

Express the event with a conditional probability or a disjoint partition.

Hint 2

Use this intermediate relation.

C_3^2/C_{t+3}^2

Worked solution

The requested value is 0.2.

Checks and common pitfalls: The unordered sample denominator counts pairs, not independent trials.

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

Teacher preparation and assessment

Choose the correct model for repeated independent trials or sampling without replacement.
Which condition is essential in binomial and hypergeometric distributions?
Which probability changes after one success is removed from a finite bag?

Defining relation: Choose the correct model for repeated independent trials or sampling without replacement.
$P(X=k)=C_n^kp^k(1-p)^{n-k}$
Conditions: Binomial trials have fixed n, common p and independence; hypergeometric sampling uses a finite population without replacement.