Quick questions on Geometric and negative binomial distributions: waiting times - Edexcel A-Level Further Maths

4short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.

What is wrong binomial coefficient?

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The negative binomial uses

\binom{x - 1}{r - 1}

, because the last trial is fixed as the

r

th success and only the first

r - 1

successes are arranged among the first

x - 1

trials.

What is q1?

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For

X \sim \operatorname{Geo}(0.25)

, find the mean. [1 mark]

What is q2?

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For the same

X

, find

P(X > 2)

. [2 marks]

What is q3?

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For a negative binomial with

r = 3

and

p = 0.5

, find the variance. [2 marks]

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