1. Note 9: Laws of Probability
The Addition Law
P(A B) = P(A) + P(B) – P(A B)
Either A or B
At least one of A or B

2. Find the probability that the student plays at least one sport.
Example 1: The probability that a student in the stats class plays cricket is The probability a student plays rugby is The probability that the student plays both sports is 0.24.
Find the probability that the student plays at least one sport.
P(C) = P(R) = P(C ∩ R) = 0.24
P(C R) = P(C) + P(R) - P(C ∩ R)
= – 0.24
= 0.91

3. Mutually Exclusive or Disjoint Events
If A and B are mutually exclusive events then: P(A B) = 0 Therefore the addition law is: P(A B) = P(A) + P(B)

4. Example 2: In a pack of 52 cards, what is the probability that a card is chosen at random will be a heart or a club?
P(Heart or club) = P(H) + P(C)
= ¼ + ¼
= ½

5. Studies:
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Exercise 16I.1