5.3.2—Probability rules
- Syllabus
- 9709–2028–2029
- Objective
- 5.3.2
- Level
- A2
Probability rules combine events without double-counting their overlap
P(A∪B)=P(A)+P(B)−P(A∩B), P(Aᶜ)=1−P(A), and P(A|B)=P(A∩B)/P(B) when P(B)>0.
Draw a Venn diagram or tree when events overlap, and identify the denominator in conditional probability before substituting numbers.
If P(A)=0.6, P(B)=0.5 and P(A∩B)=0.2, then P(A∪B)=0.9.
Adding P(A) and P(B) without subtracting overlap can exceed 1.
ConceptA-Level CAIE Mathematics A2