Complement Rule

1. Definition: The complement of an event A, denoted as A', is the event that A does not occur.
2. Complement Rule: The probability of the complement of an event A happening is equal to 1 minus the probability of the event A happening.
3. Mathematical Expression: P(A') = 1 - P(A)
4. Example: If the probability of rolling a 6 on a fair six-sided die is 1/6, then the probability of not rolling a 6 (complement of the event) is 1 - 1/6 = 5/6.

Understanding the complement rule is crucial in probability calculations and forms the basis for many probability problems.