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Date November 2016 Marks available 10 Reference code 16N.3sp.hl.TZ0.4
Level HL only Paper Paper 3 Statistics and probability Time zone TZ0
Command term Prove that Question number 4 Adapted from N/A

Question

Two independent discrete random variables X and Y have probability generating functions G(t) and H(t) respectively. Let Z=X+Y have probability generating function J(t).

Write down an expression for J(t) in terms of G(t) and H(t).

[1]
a.

By differentiating J(t), prove that

(i)     E(Z)=E(X)+E(Y);

(ii)     Var(Z)=Var(X)+Var(Y).

[10]
b.

Markscheme

J(t)=G(t)H(t)    A1

[1 mark]

a.

(i)     J(t)=G(t)H(t)+G(t)H(t)     M1A1

J(1)=G(1)H(1)+G(1)H(1)    M1

J(1)=G(1)+H(1)    A1

so E(Z)=E(X)+E(Y)     AG

(ii)     J     M1A1

J''(1) = G''(1)H(1) + 2G'(1)H'(1) + G(1)H''(1)

= G''(1) + 2G'(1)H'(1) + H''(1)    A1

{\text{Var}}(Z) = J''(1) + J'(1) - {\left( {J'(1)} \right)^2}    M1

= G''(1) + 2G'(1)H'(1) + H''(1) + G'(1) + H'(1) - {\left( {G'(1) + H'(1)} \right)^2}    A1

= G''(1) + G'(1) - {\left( {G'(1)} \right)^2} + H''(1) + H'(1) - {\left( {H'(1)} \right)^2}    A1

so {\text{Var}}(Z) = {\text{Var}}(X) + {\text{Var}}(Y)     AG

 

Note: If addition is wrongly used instead of multiplication in (a) it is inappropriate to give FT apart from the second M marks in each part, as the working is too simple.

 

[10 marks]

b.

Examiners report

[N/A]
a.
[N/A]
b.

Syllabus sections

Topic 7 - Option: Statistics and probability » 7.1 » Cumulative distribution functions for both discrete and continuous distributions.
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