# Probability Rules Rap

1. The event `Ac` ("not A") is called the of event `A`. 2. `P(A)` + `P(Ac)` sums to: .
3. If two events are independent, they do not:   .

Whether a person is registered as a Democrat or whether that person is registered as a Republican are disjoint. Whether a randomly chosen person is registered as a Democrat and whether a second randomly chosen person is registered as a Republican are independent.
4. If two events `A` and `B` are independent, the probability that they both happen is equal to the of `P(A)` and `P(B)`.

If the events `A` and `B` are not independent, then the probability that they both happen equals `P(B)` times: .

P(C|D) is notation for the probability that the event C happens when you know that event D has occurred.
6. If two events are disjoint (i.e., mutually exclusive), they do not: .

Whether a person is registered as a Democrat or whether that person is registered as a Republican are disjoint. Whether a randomly chosen person is registered as a Democrat and whether a second randomly chosen person is registered as a Republican are independent.
7. The union of events `A` or `B` is often described using the words "A or B" (A ∪ B). This means that events `A` or `B` happened. Assume events `A` and `B` are disjoint (i.e., mutually exclusive). To find the probability that at least one happens,    `P(A)` and `P(B)`.
Now suppose events `A` and `B` do have overlap. To find `P(A ∪ B)`, you take `P(A)` + `P(B)` and then the probability of the overlap. 