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, p≠0.
Show that P(A∪B)=2p−p2.
Find P(A|A∪B) in simplest form.
Markscheme
P(A∪B)=P(A)+P(B)−P(A∩B)
=P(A)+P(B)−P(A)P(B) (M1)
=p+p−p2 A1
=2p−p2 AG
[2 marks]
P(A|A∪B)=P(A∩(A∪B))P(A∪B) (M1)
Note: Allow P(A∩A∪B) if seen on the numerator.
=P(A)P(A∪B) (A1)
=p2p−p2 A1
=12−p A1
[4 marks]
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∩(A∪B)). Large numbers of fully correct answers were seen.
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∩(A∪B)). Large numbers of fully correct answers were seen.