Probability complement rule: P(A') = 1 - P(A) where A' is complement (not A). Key concepts: all possibilities covered by event and complement, probabilities sum to 1. Common traps: confusing complement with other event operations, calculation errors. Exam tips: identify complement carefully from problem. Time-saving: recognize when complement simpler than direct calculation. Applications: probability of failure, non-occurrence scenarios. Example: if P(rain) = 0.3, then P(no rain) = 0.7. Properties: events A and A' are mutually exclusive and exhaustive. Verification: P(A) + P(A') = 1 always holds. Understanding complements essential for probability. Practice identifying complements in various contexts.