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Date May 2016 Marks available 2 Reference code 16M.1.hl.TZ2.7
Level HL only Paper 1 Time zone TZ2
Command term Show that Question number 7 Adapted from N/A

Question

A and B are independent events such that P(A)=P(B)=p, p0.

Show that P(AB)=2pp2.

[2]
a.

Find P(A|AB) in simplest form.

[4]
b.

Markscheme

P(AB)=P(A)+P(B)P(AB)

=P(A)+P(B)P(A)P(B)    (M1)

=p+pp2    A1

=2pp2    AG

[2 marks]

a.

P(A|AB)=P(A(AB))P(AB)    (M1)

 

Note:     Allow P(AAB) if seen on the numerator.

 

=P(A)P(AB)    (A1)

=p2pp2    A1

=12p    A1

[4 marks]

b.

Examiners report

Part (a) posed few problems. Part (b) was possibly a good discriminator for the 4/5 candidates. Some were aware of an alternative (useful) form for the conditional probability, but were unable to interpret P(A(AB)). Large numbers of fully correct answers were seen.

a.

Part (a) posed few problems. Part (b) was possibly a good discriminator for the 4/5 candidates. Some were aware of an alternative (useful) form for the conditional probability, but were unable to interpret P(A(AB)). Large numbers of fully correct answers were seen.

b.

Syllabus sections

Topic 5 - Core: Statistics and probability » 5.3 » Combined events; the formula for P(AB) .

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