Event notation and regions
Translate the conditions first. A diagram supplies counts; notation selects regions
- Intersection Keep items satisfying both conditions \(\displaystyle A\cap B\)
- Union Keep items in at least one set, including the overlap \(\displaystyle A\cup B\)
- Complement A prime means everything in the stated universe outside that set \(\displaystyle A'=S\setminus A\)
- Only and neither A only is in A but outside B. Neither is outside both \(\displaystyle \begin{gathered}A\text{ only}:A\cap B'\\\text{neither}:A'\cap B'\end{gathered}\)
- Counts and probabilities The first line shows three ways to write a count. For uniform selection, divide the event count by the universe count \(\displaystyle \begin{gathered}n(E),\;N(E),\;|E|\\\Pr(E)=\frac{n(E)}{n(S)}\end{gathered}\)
- No common outcomes Mutually exclusive events have an empty intersection. The empty set contains no elements; it is not the number zero \(\displaystyle A\cap B=\varnothing\)
Worked example
A only has 7 items, B only 5, both 3 and neither 9. Select one of the 24 items uniformly. Find the probability of exactly one property
- \(\displaystyle 7+5=12\)Add the two disjoint only regions
- \(\displaystyle \Pr(\text{exactly one})=\frac{12}{24}=\frac12\)Divide by the whole universe
Common mistakes
Difficulty levels
Questions about this topic
Does or include both?+
Yes. A union B includes both unless the wording explicitly says exactly one
What does the prime apply to?+
It complements the immediately preceding set or bracketed expression within the stated universe
Is outside the overlap the same as neither?+
No. Outside the overlap includes A only and B only as well as neither
What does a shaded option mean?+
Shading selects the event region. Unshaded regions are excluded; circle size does not measure probability
Why is the empty set different from zero?+
The empty set contains no objects. Zero describes how many objects it contains
Can I use a calculator?+
A calculator is allowed unless a question is marked no calculator
Which year level and chapter is this?+
Year 9 Mathematics, chapter 9: Probability and statistics