Grade 11 Probability

The addition identity, independent events, Venn diagrams, contingency tables and tree diagrams.

Gr 11 Term 3 - ProbabilityTerm 3 - 2 weeks (CAPS)

Practise Probability in the app →

What gets asked

• Use $P(A\text{ or }B)=P(A)+P(B)-P(A\text{ and }B)$.
• Test whether two events are independent.
• Fill in a Venn diagram or contingency table and read probabilities from it.
• Use a tree diagram for two draws, with or without replacement.

You must be able to

• Subtract the overlap exactly once when combining with 'or'.
• Independent means $P(A\text{ and }B)=P(A)\times P(B)$ - test it, never assume it.
• Start a Venn diagram from the intersection and work outwards.
• Without replacement, the second branch's denominator drops by one.

Traps that cost marks

• Calling mutually exclusive events independent. They are opposites in spirit: exclusive events CANNOT both happen, so the product test fails.
• Keeping the same denominator on the second draw without replacement. One object is gone: 10 becomes 9 in the second branch.
• Adding along a tree branch and multiplying across paths. Multiply ALONG a path, add the qualifying paths.

Worked example